Genetics is a core biology concept that deals with heredity and variation in living organisms. It explains how genetic information passes from generation to generation. Knowledge of genetics and related subjects, such as DNA structure and function, gene expression, genetic disorders, inheritance patterns, and genetic variation, is examined on the MCAT. So, in order to do well on the MCAT, your MCAT study schedule must include this high-yield MCAT topic. This blog will comprehensively cover the major genetics concepts so you can get your desired MCAT score!

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Article Contents
13 min read

Phenotype and genotype Gene, allele, and locus Dominance Hybridization Gene pool Sex-linked characteristics Mutation and its types Hardy-Weinberg principle Test cross Gene mapping

Phenotype and genotype

The term "phenotype" describes an organism's observable features, such as its physical attributes, behavioral patterns, and physiological processes. For example, an individual's phenotype includes eye color, height, and hair structure. Within a species, phenotype can differ significantly between people.

Genotype describes an organism's genetic makeup, including its DNA sequence and any differences therein. In addition, it determines the inherited characteristics of an organism, including its blood type, eye color, and susceptibility to specific illnesses. Several variables, such as genetics, environment, and mutations, can all impact phenotypes and genotypes. In summary, phenotype describes an organism’s observable traits, whereas genotype describes the genetic material underlying those traits.

Gene, allele, and locus

A gene is a heredity unit that passes genetic information from generation to generation. Genes are composed of DNA, which directs an organism's growth, development, and functionality. Each gene has a promoter, coding section, and terminator. In addition, genes produce the RNA or protein that determines a particular characteristic of an organism.

A gene has different variants called alleles. These are one or more alternate forms of a gene that may be found at the exact chromosomal location or region. Genes with only two possible variants are referred to as single alleles. For example, Mendel's pea plants have two variant seed forms, round (R) and wrinkled (r). Human blood groups A, B, AB, and O are the most common example of multiple alleles, where the gene determining the blood groups has more than two variants.

Two copies of each gene, one from each parent, are passed down to each person. These versions could either be distinct (heterozygous) or identical (homozygous). The terms "homozygous" and "heterozygous" describe a person's genome, or genetic makeup, for a specific gene. For example, a person is homozygous for a trait if both alleles at a particular locus are identical, such as RR or rr. Conversely, an individual is considered heterozygous for a gene when the two alleles at a locus differ, such as Rr.

A recessive allele is an allele that is only expressed as a phenotype when an individual has two copies of that allele. If a person carries one recessive gene, the other dominant allele will mask it. For example, in humans, the allele for brown eyes is recessive to the allele for blue eyes. Accordingly, a person with two copies of the blue eye allele (bb) will have blue eyes, whereas a person with at least one copy of the brown eye allele (Bb or BB) will have brown eyes. This is due to the recessive blue eye gene's expression masked by the dominant brown eye allele.

Contrary to the mutant type, the most prevalent allele of a specific gene found in a population is the wild-type allele. This is because it produces the normally expected phenotype most commonly observed in nature. For instance, the wild-type allele for eye color in fruit flies determines red eyes, whereas a mutated allele can result in other eye colors like white or purple. The wild-type genes can either be dominant or recessive. For instance, although some forms of dwarfism in humans are dominant traits, they are still regarded as mutations because they do not affect most of the population. An allele is regarded as the wild type whenever it constitutes a majority (more than 50%).

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Dominance: complete, incomplete, codominance, leakage, penetrance, expressivity

Different patterns of inheritance for traits governed by genes include complete dominance, incomplete dominance, codominance, leakage, penetrance, and expressivity.

In complete dominance, one allele (dominant) completely masks the effect of the other allele (recessive). For example, the gene for human brown eyes is completely dominant over the blue eye gene. Incomplete dominance, on the other hand, is a gene interaction wherein a gene's two alleles at a locus are partially expressed, resulting in an intermediate heterozygote phenotype. Neither allele completely dominates the other. Wavy hair in humans is an example of incomplete dominance. Straight (HH) and curly hair (hh) are partially dominant variants of the same gene. So, a person having both alleles (one for straight and one for curly hair) will have wavy hair (Hh), a blend of the two.

When both alleles are expressed fully and equally in a heterozygous condition, then codominance results. Neither allele is blended; it is completely expressed. The human blood type inheritance is a typical example of codominance. The O gene is recessive, whereas blood types A and B are codominant. Both A and B alleles express equally in a person having blood type ‘AB’.

Sometimes, a normally recessive trait is expressed in a heterozygous phenotype. It happens when a few non-dominant genes "leak through" and manifest themselves in the phenotype. For example, in snapdragons, the gene for red flowers (RR) dominates over the allele for white flowers (ww). But occasionally, a heterozygous flower (Rw) may display white coloration, demonstrating that the recessive gene has "leaked" through.

Another phenomenon is called penetration, which describes the percentage of individuals with a given genotype exhibiting the predicted phenotype. In the case of a known disease-causing gene, the penetrance of that gene is the percentage of people who suffer from the disease. The expression level of a gene in the phenotype is referred to as expressivity. It can differ between people who share the same genotype, leading to differences in the type or severity of a specific trait. For instance, one individual with the same genotype may experience mild symptoms from a genetic disorder, while another may experience severe symptoms, indicating differences in expressivity.

Hybridization: viability (ability to survive and reproduce)

The mating of two individuals from distinct species or subspecies to produce offspring with mixed genetic traits (hybrids) is known as hybridization. Closely related species hybrids usually die or are infertile if they do survive. Offspring from hybrids are often less fertile or can’t survive. Mules, for instance, are a hybrid of a horse and a donkey, but they are sterile and incapable of reproduction. Hybridization can result in new genetic combinations and the diversification of species. Still, the viability of hybrid offspring can differ depending on the mating characteristics and the environment in which they exist.

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Gene pool

The total genetic information, or alleles, existing in a population of interbreeding organisms is referred to as the gene pool. It incorporates various genetic variations in a population's members, including dominant and recessive genes and mutations that develop over time. Several factors, such as genetic drift, gene flow, mutation, natural selection, and human activities like artificial selection and genetic engineering, can impact the size and makeup of the gene pool.

Meiosis and other factors affecting genetic variability

Meiosis, a cell division, is necessary for sexual reproduction in eukaryotic species. Meiosis is significant because it can produce genetic variation through the independent recombination of homologous chromosomes during meiosis I and II. Additionally, meiosis ensures that the number of chromosomes in the resulting gametes reduces by half, necessary for the zygote's proper fertilization and growth.

Mitosis is another type of cell division; both divisions differ significantly. Meiosis produces gametes (sperm and egg cells) for sexual reproduction, whereas mitosis involves tissue growth and repair. Chromosome number in daughter cells is another significant difference between the two. Mitosis creates two identical daughter cells with the same number of chromosomes as the parent cell Whereas, meiosis results in four distinct progeny cells with half chromosome number. Meiosis also brings genetic diversity through crossing over (exchange of DNA segments between homologous chromosomes) and independent assortment, whereas mitosis doesn’t involve any such mechanism.

Segregation and independent assortment of alleles

In meiosis, alleles are segregated during gamete formation, which is called segregation. This ensures that each gamete gets only one gene variant. However, when homologous chromosomes are randomly distributed during meiosis-I, maternal and father chromosomes are assorted randomly into the resulting gametes. This process is known as independent assortment. Resultantly, unique allele combinations are produced, increasing genetic variety.

Genes on the same chromosome may be linked and inherited as a unit rather than independently, which is called linkage. Genes that are located distantly on the chromosome are likely to undergo recombination and inherit independently. Recombination occurs during prophase-I when homologous chromosomes’ non-sister chromatids physically exchange DNA fragments (crossing over). It allows the creation of genetically diverse gametes, crucial for sexual reproduction and a species' existence.

When two homologous chromosomes cross over at a single point, it is called a single point cross-over. Only the DNA strands near the crossover point share genetic material. As a consequence, there are two non-recombinant and two recombinant chromosomes. Double crossovers occur when two homologous chromosomes' chromatids exchange segments at two points. The synaptonemal complex (protein structure) develops between homologous chromosomes. It is crucial for preserving the genetic and physical connections and alignment and matching of homologous chromosomes. Any defect in the synaptonemal complex can result in mistakes in the segregation of chromosomes during meiosis, which can cause genetic diseases or infertility.

When homologous chromosomes pair up and synapse, they form a bivalent structure. Four c is crucial for preserving the genetic and physical connections and aligning, called tetrads. Tetrad formation is vital for effective chromosome separation during meiosis.

Sex-linked characteristics: X and Y linked

Characteristics found on sex-determining or sex chromosomes are referred to as sex-linked or sex-linkage qualities. Genes on X and Y chromosomes (sex chromosomes) determine these traits. In humans, males have one X, and one Y chromosome (XY), while females have two X chromosomes (XX). Because men only have one copy of the X chromosome, they are more likely to exhibit traits found on the X chromosome. Colorblind and hemophilia are examples of X-linked traits. These diseases are more common in men because they only require one copy of the mutated gene to manifest the disorder. However, for the disorder to appear in females (having two X chromosomes), two versions of the mutated gene must be inherited. An incomplete or missing X chromosome can cause some sex-related characteristics in women, such as Turner syndrome. Females in this situation only have one X chromosome, which can cause many physical and developmental anomalies.

Some traits are Y-linked; genes on the Y chromosome control them. These traits are expressed in males only because females have no Y chromosome. Male-pattern baldness is an example of a Y-linked trait. The Human Y chromosome is much smaller than X chromosome and contains fewer genes. Compared to the X chromosome (have thousands of genes), only a few dozen genes are present on the Y chromosome, which play a role in the development and sex determination. For instance, the SRY gene on the Y chromosome determines maleness.

Sex determination

An organism's sexual characteristics are developed under the control of the sex-determination system. Sex chromosomes define sex in humans. Early in embryonic development, the expression of the SRY gene initiates the development of male-specific characteristics, while its absence stimulates the development of the ovaries. In humans, there is a XX-XY type of sex determination. Males typically have one X and one Y chromosome in human beings (as well as many other animals), so they are heterogametic (two different chromosomes), whereas females usually have two X chromosomes.

Grasshopper species also employ the XX/XO single-chromosome sex classification scheme, where males are classified as XO because they possess only one sex chromosome. As a result, because they produce two distinct types of gametes, males are the heterogametic sex. The pattern is reversed in butterflies and moths (order Lepidoptera): females are the heterogametic (ZW) sex, while males are homogametic (ZZ). Without the W chromosome, ZZ grows into males and ZO into females.

Mutation and its types

A mutation is a variation in a gene's or a chromosome's DNA sequence. Mutations occur naturally due to mistakes in DNA duplication or repair, or external agents like radiation, chemicals, or viruses can cause them.

Random mutations

Mutations that happen by chance, without a known reason or outside influence, are known as random mutations. They can develop due to mistakes in DNA replication, radiation or chemical exposure, or just by coincidence. These abnormalities can be transmitted to the next generation.

Transcription and translation error

The transcription error occurs when RNA polymerase makes any mistake while copying the DNA into mRNA. When the ribosome decodes the mRNA sequence to produce a protein translation error may occur. Various factors can contribute to translation errors, such as changes in the DNA sequence, tRNA molecule mistakes, or ribosome errors. A translation error can produce a protein with an amino acid sequence distinct from the one coded by the DNA sequence.

Base substitution, inversion, deletion, addition, translocation, and mispairing

One nucleotide is substituted for another in base pair mutation. In other cases, a DNA fragment may be reversed in orientation (inversion), or an extra nucleotide is added (addition) or deleted (deletion) from the DNA sequence. In other mutations, a DNA segment may be moved from one location to another in the genome, called translocation or mispairing occurs when two non-complementary nucleotides pair up.

However, not all these mutations are detrimental or beneficial. Some may not impact protein function or are advantageous, while others may disrupt the gene function, alter the protein’s amino acid sequence and affect normal cell functioning. Advantageous proteins increase the likelihood of an organism’s survival and reproduction. For instance, a mutation that provides resistance to an organism against a disease-causing agent is beneficial. Deleterious mutations, on the other hand, harm an organism and reduce its possibilities of survival and reproduction. Some mutations occur in the genes that produce proteins or enzymes involved in metabolism. These lead to disorders called inborn metabolism errors that impact how the body processes specific nutrients or substances. Protein or enzyme deficiency or malfunction can cause the buildup of some importance in the body or a deficiency of essential substances, resulting in various health issues.

Agents that can alter DNA are known as mutagens. These agents may be chemical—like some compounds in tobacco smoke or industrial pollutants—or physical—like radiation. A higher chance of developing cancer can result from mutations in specific genes; mutagens that do so are referred to as carcinogens. Carcinogens can alter DNA and other genetic material. These modifications may result in abnormal cell division and development, a sign of cancer.

Genetic drift

Random chance events may change the frequency of an allele in a population, called genetic drift. Over time, it brings changes in the population's genetic composition. For example, it may lead to the total eradication of gene variants, reducing genetic diversity. On the other hand, it can also cause the frequency of that allele to rise and possibly become fixed in the community. Small populations are more susceptible to genetic drift because random events can alter the population's genetic composition more significantly. Large people can also experience it, but the effects are not pronounced.

Hardy-Weinberg principle

G. H. Hardy (1908) and Wilhelm Weinberg (1909) independently proposed the Hardy-Weinberg theorem. This principle describes the relationship between allele and genotype frequencies in a population that is not evolving. The Hardy-Weinberg rule states that in a large population of randomly mating individuals, the frequencies of alleles and genotypes at a specific locus will not change from generation to generation, assuming that the population is unaffected by mutation, migration, natural selection, or genetic drift. It is based on the idea that the population's alleles exist in a stable equilibrium.

According to the principle, it is possible to determine an allele's frequency in a population as the square of the frequency of the corresponding genotype. To illustrate, if the homozygous dominant genotype (AA) has a frequency of p and the homozygous recessive genotype (aa) has a frequency of q. Then, the frequency of the dominant allele (A) will be p + q, and the frequency of the recessive allele (a) will be q. The Hardy-Weinberg principle serves as a baseline to quantify changes in allele and genotype frequencies over time and identify evolutionary forces on a population.

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Test cross: backcross; F1 and F2 parental generations

A testcross (a backcross) is a genetic cross to identify an individual's genotype for a specific characteristic. It implies mating a homozygous recessive individual with a test individual (having an unknown genotype). The progeny produced can provide information about the test subject's genotype. The homozygous recessive individual is the "tester" in a testcross as it will only pass on recessive genes to the progeny. If the test subject is heterozygous, half of the children will have the recessive allele, and the other half will have the dominant allele. All progeny will bear the dominant allele if the test subject is homozygous dominant.

When two heterozygous individuals are crossed (F1 generation), the testcross is often employed to identify the genotype of the resulting individual. A cross between two heterozygous people will produce a heterozygous child for the desired trait. The offspring produced from a testcross between one of the F1 individuals and a homozygous recessive individual can reveal the genotype of the F1 individual. The individuals initially crossed in a breeding experiment are known as the parental generation (P generation). The first filial generation (F1 generation) is their progeny, which, when crossed, produces the second filial generation (F2 generation).

Gene mapping: frequency crossovers

The process of locating genes on chromosomes is known as gene mapping. One of the methods used for gene mapping is based on the frequency of crossovers between genes. Centimorgans (cM) are used to quantify the frequency of crossovers. One cM corresponds to a 1% exchange frequency. On a chromosome, two genes near one another have lower crossover probabilities and recombination frequencies. On the other hand, if two alleles are far apart, they will recombine frequently and are more likely to go through a crossover.

Biometry, the application of statistical techniques, is crucial for gene mapping. Chi-squared tests, likelihood ratio tests, and maximal likelihood estimation are a few of the statistical methods used in biometry for gene mapping. These techniques can be used to evaluate theories regarding the distribution of genes on chromosomes and the frequency of gene crossovers.

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