Genetics for the DAT
/Learn key DAT concepts about genetics, plus practice questions and answers
everything you need to know about genetics for the dat
Table of Contents
Part 1: Introduction to genetics
Part 2: Classical genetics
a) Chromosomes
b) Laws of Mendelian genetics
c) Dominance and recessivity
d) Punnett squares
Part 3: Human genetics
a) Sex-linked genetics
b) Exceptions to complete dominance
c) Genetic linkage
d) Extranuclear inheritance
Part 4: Epigenetics
a) Histone structure and function
b) X inactivation
Part 5: Genomics
Part 6: High-yield terms
Part 7: Questions and answers
----
Part 1: Introduction to Genetics
Genetics is the study of heredity and variation in living organisms. Understanding genetics provides insight into the mechanisms underlying traits, diseases, and evolutionary processes. From Mendel's groundbreaking experiments with pea plants to modern advancements in molecular genetics, the field continues to unravel the complexities of inheritance and gene expression. This guide will prepare you for the DAT by covering all important topics related to genetics. As you study, pay attention to bold, high-yield terms and test yourself with practice questions and answers at the end.
----
Part 2: Classical genetics
a) Chromosomes
In the majority of cells, genetic information is housed as DNA arranged into chromosomes. Prokaryotes, which encompass archaea and bacteria, lack membrane-bound organelles. In these organisms, DNA typically forms a circular molecule and resides unprotected in the cytoplasm - they lack a true nucleus!
Eukaryotes, on the other hand, have membrane-bound organelles. This category encompasses animals, plants, fungi, and protists, including humans. Eukaryotic cells hold linear DNA chromosomes supported by proteins. This DNA is housed within the nucleus of the cell, with each chromosome comprising a single DNA molecule.
FIGURE 1: PROKARYOTIC AND EUKARYOTIC CHROMOSOMES
Euchromatin refers to DNA that is actively transcribed or in a relaxed state. Heterochromatin, on the other hand, is DNA that is transcriptionally inactive or condensed. In mature eukaryotic cells, chromosomes consist of pairs of chromatids connected at a centromere. Each chromatid possesses a long and short arm. Telomeres are regions that contain high G-C content located at the ends of each chromatid. These regions serve as protective buffers against DNA shortening during replication, and they shorten with each round of cell replication. Once the cell reaches a certain number of divisions, known as the Hayflick limit, it can no longer safely replicate and will undergo cell death.
FIGURE 2: STRUCTURE OF CHROMOSOMES
Karyotyping is a laboratory technique used to visualize an organism's chromosomes. This process typically involves staining the chromosomes under a microscope, sometimes using fluorescent dyes to distinguish between different chromosomal regions. Photographs of the condensed chromosomes are then taken and digitally manipulated to arrange them in order of decreasing size.
FIGURE 3: STEPS OF KARYOTYPING
Karyotyping serves as a valuable tool in identifying chromosomal irregularities associated with diseases. These abnormalities include aneuploidy, characterized by the absence or presence of an extra chromosome, and structural anomalies, involving alterations in individual chromosomes. For instance, aneuploidy conditions like trisomy 21, commonly known as Down syndrome, involve the inheritance of an additional copy of chromosome #21. Structural abnormalities manifest in diverse forms, such as:
Deletion: in which a segment of a chromosome is missing
Duplication: in which a segment of a chromosome is repeated on the same arm
Inversion: in which two regions on the same chromosome are swapped
Substitution: in which a region on one chromosome is inserted into the arm of another chromosome
Translocation: in which the terminal regions of two different chromosomes are swapped
Additional changes in DNA, also known as a mutation, can occur on a smaller scale. Mutations generally arise as a result of the presence of mutagens in the environment.
A point mutation is a result of the change of a single nucleotide within the DNA sequence. The nucleotide (A, C, T, or G) may be swapped out for another nucleotide at the same location.
A frameshift mutation is a result of either a deleted or inserted set of nucleotides that alter the reading frame of the sequence. (Recall that three-letter codons are translated by the ribosome into amino acids; changing the location of the frame can alter the translated amino acids.)
A mispairing mutation results from the separation and failed re-attachment of two strands of DNA. This mutation results in the mispairing of individual DNA nucleotides: for example, A failing to bind with T or C failing to bind with G.
The same locus or gene can yield various phenotypes. A single gene responsible for eye color, for example, may produce multiple eye colors, such as blue, brown, or green. This type of variability is termed polymorphism.
Typically, humans possess 23 pairs of chromosomes in each cell. One set is contributed by each parent, resulting in a total of 46 chromosomes in a normal human karyotype. The human genome comprises 23 pairs of homologous chromosomes, meaning that corresponding paternal and maternal chromosomes carry identical sets of genes. Humans, like most mammals, are diploid organisms, meaning they have two sets of chromosomes in each cell.
A minor deviation from this pattern is observed in the 23rd pair of chromosomes, known as the sex chromosomes. These chromosomes determine an individual's genetic sex. Females have two X chromosomes (XX), while males have one X chromosome and one smaller Y chromosome (XY). The remaining 44 chromosomes are referred to as autosomes and are numbered from pairs 1 to 22.
FIGURE 4: AN EXAMPLE OF A HUMAN KARYOTYPE
If all humans have the same set of chromosomes that carry the same sets of genes, how do our individual differences arise? One of the most important sources of variation lies in the concept of alleles, or gene variants.
While members of the same species have the same genes, these genes can vary in DNA sequence between individual members and even between the two copies within a diploid cell. Multiple alleles can exist for a single gene, each with potentially different characteristics. Each human cell carries at least 2 alleles of each gene, and these can either be identical or unique.
While all alleles for a gene are found at the same position, or locus (plural: loci) on a chromosome, variations in gene sequences confer different characteristics for gene activity. It is this complex interplay between the different genetic profiles of alleles, or genotypes, that leads to the different physical characteristics, or phenotypes, of different individuals.
b) Laws of Mendelian genetics
In the 19th century, a monk named Gregor Mendel conducted groundbreaking studies on genetic inheritance using pea plants. Mendel formulated several laws on trait heredity, which have been refined and verified by modern research. Some of Mendel’s laws govern the processes that take place during meiosis, a crucial stage in sexual reproduction where haploid gametes are produced.
The law of segregation states that in a diploid organism, the two allelic copies of genes are equally separated into gametes. This ensures that each gamete receives one of the allele pairs. Since diploid organisms possess two gene copies, this principle applies to all genes during meiosis I, ensuring the fair distribution of alleles to daughter gametes from the dividing mother cell.
The law of independent assortment asserts that the segregation of alleles for one gene happens independently of the segregation of alleles for another gene. Consequently, different genes transmit alleles to gametes without influencing the assortment of other genes. The independent arrangement of homologous chromosomes before anaphase allows alleles to be distributed differently among their daughter cells, contributing to genetic variation within the gametes.
FIGURE 5: THE LAW OF SEGREGATION AND LAW OF INDEPENDENT ASSORTMENT
c) Dominance and recessivity
Remember that an individual’s genotype refers to the genetic alleles responsible for a trait, while its phenotype is the observable expression of that trait. When a diploid organism carries two identical alleles of a gene, it is termed homozygous. On the other hand, the presence of two different alleles indicates heterozygosity for that gene. The interaction between these alleles determines the phenotypic expression of the genes.
Alleles can be categorized as dominant or recessive. Dominant alleles exert a stronger phenotypic effect and will mask the effects of recessive alleles when both are present in a heterozygote. This implies that to express a recessive phenotype, the organism must possess two copies of the recessive allele (i.e., a homozygous recessive genotype). Conversely, the dominant phenotype could manifest with just one copy of the allele or two (i.e., a heterozygous or homozygous dominant genotype). When a phenotypic trait is expressed with only one inherited copy of a dominant allele, it exhibits complete dominance.
A letter-based shorthand is often used to represent dominant or recessive alleles. Typically, dominant alleles are denoted by capital letters (like R or XR), while recessive alleles are indicated by lowercase letters (r or Xr).
| Inherited alleles | Genotype | Phenotype | ||
|---|---|---|---|---|
Remember that genes encode proteins, and these proteins dictate function within an organism. Consequently, a recessive gene often leads to a loss of function in the corresponding protein. In many cases, possessing two recessive alleles can be problematic, potentially resulting in pathological conditions. This is exemplified by diseases arising from a homozygous recessive genotype, such as cystic fibrosis, caused by mutations in the CFTR gene. Individuals with two mutated (recessive) alleles experience cystic fibrosis, whereas those with one normal allele (wild-type) and one recessive allele do not manifest the condition due to the dominant wild-type allele's suppressive effects. Instead, such heterozygous individuals are termed carriers of the disease, capable of transmitting the recessive allele to their offspring without exhibiting the recessive phenotype themselves.
d) Punnett squares
A Punnett square is used to estimate the genotypic and phenotypic makeup of offspring resulting from the crossing of two genotypes. The simplest form is a monohybrid cross, which uses a 2x2 Punnett square to map out the potential combinations of parental alleles with respect to one gene. Imagine that in a cross between Parents 1 and 2, Parent 1 carries the alleles Aa and Ab, while Parent 2 carries the alleles Ac and Ad. Each gamete produced by a parent carries one of these two alleles. Thus, a gamete from Parent 1 may contribute either allele Aa or Ab, while a gamete from Parent 2 may contribute either allele Ac or Ad. Thus, when each parent contributes one allele in a gamete, it is possible to produce offspring with one of four different genotypes: Aa Ac , AaAd, AbAc, and AbAd. An offspring can inherit any of these genotypes with equal probability, at a 1 in 4 chance.
FIGURE 6: MONOHYBRID CROSS
A dihybrid cross introduces complexity by involving alleles for two distinct genes instead of just one. Since each parent can contribute one of two alleles per gene in a gamete, they can each produce 2 x 2 = 4 potential gametes for two genes. Consequently, a 4x4 Punnett square is employed, offering 16 possible outcomes.
Consider a dihybrid cross where Parent 1 possesses the AaBb genotype, as does Parent 2.
Parent 1 possesses two different alleles (A and a) at the first gene locus, meaning a gamete formed by Parent 1 will contain one of these alleles (A or a). Similarly, at the second gene locus, Parent 1 carries two distinct alleles (B and b), resulting in a gamete with one of these alleles (B or b).
The same analysis applies to Parent 2. With two different alleles (A and a) at the first gene locus, a gamete from Parent 2 will carry one of these alleles (A or a). Likewise, at the second gene locus, Parent 2 harbors two different alleles (B and b), leading to a gamete containing one of these alleles (B or b).
FIGURE 7: AN EXAMPLE OF DETERMINING THE FOUR POSSIBLE GAMETES CONTRIBUTED BY A SINGLE PARENT
After determining the possible gametes contributed by each parent, the Punnett square is created. The possible gametes are written across the top row and leftmost column, and genotypes of individual offspring can be written by combining the genotypes of gametes from each parent.
FIGURE 8: AN EXAMPLE OF A DIHYBRID CROSS
The frequencies of individual genotypes resulting from the Punnett square can be used to predict the frequencies of genotypes in the offspring. In this example:
The genotype AABB appears once. Thus, the probability of randomly receiving an offspring that is homozygous dominant in both gene A and gene B is 1/16.
The genotype AABb appears twice. The probability of randomly receiving an offspring that is both homozygous dominant in gene A and heterozygous in gene B is 2/16, or 1/8.
The genotype AaBB appears twice. The probability of randomly receiving an offspring that is both heterozygous in gene A and homozygous dominant in gene A is 2/16, or 1/8.
The genotype AAbb appears once. Thus, the probability of randomly receiving an offspring that is both homozygous dominant in gene A and homozygous recessive in gene A is 1/16.
The genotype AaBb appears four times. Thus, the probability of randomly receiving an offspring that is heterozygous in both genes is 4/16, or 1/4.
The genotype Aabb appears two times. Thus, the probability of randomly receiving an offspring that is both heterozygous in gene A and homozygous recessive in gene B is 2/16, or 1/8.
The genotype aaBB appears once. The probability of randomly receiving an offspring that is both homozygous recessive in gene A and homozygous dominant in gene B is 1/16.
The genotype aaBb appears twice. Thus, the probability of randomly receiving an offspring that is both homozygous recessive in gene A and heterozygous in gene B is 2/16, or 1/8.
The genotype aabb appears once. Thus, the probability of randomly receiving an offspring that is homozygous recessive in both gene A and gene B is 1/16.
Note that to predict the frequency of expressed traits in the offspring, these genotypes must be decoded to their respective phenotypes. In this example,
The probability of receiving an offspring that is dominant in traits encoded by both gene A and gene B is 9/16.
The probability of receiving an offspring that is both dominant in a trait encoded by gene A and recessive in a trait encoded by gene B is 3/16.
The probability of receiving an offspring that is both recessive in a trait encoded by gene A and dominant in a trait encoded by gene B is 3/16.
The probability of receiving an offspring that is recessive in traits encoded by both gene A and gene B is 1/16.
Punnett squares serve as tools for predicting the potential offspring genotype resulting from the crossing of two organisms with known genotypes. Conversely, they can also be applied in a test cross scenario to ascertain the genotype of an organism. Also known as a back cross, a test cross involves mating the organism under investigation with a homozygous recessive individual. This aids in determining whether the organism is a dominant homozygote or a heterozygote by observing the proportion of offspring displaying dominant or recessive phenotypes.
For instance, let's consider a test cross involving flowers where blue color represents the dominant phenotype and white color represents the recessive phenotype. To discern the unknown genotype of a blue flower, it is paired with a homozygous recessive (white-colored) individual. Following the breeding of these two parents, the offspring's phenotypes are tallied and compared with predictions derived from Punnett squares.
FIGURE 9: TEST CROSS EXAMPLE
Test crosses can be conducted between individuals across successive generations. Here, the initial parental (P) generation gives rise to the F1 ("first filial") generation. Subsequent crosses involving individuals from the F1 generation produce the F2 ("second filial") generation.
----
Gain instant access to the most digestible and comprehensive DAT content resources available. Subscribe today to lock in the current investments, which will be increasing in the future for new subscribers.