ATI TEAS GUIDE TO SCIENCE | UNDERSTANDING GENETICS

ATI TEAS SCIENCE REVIEW – GENETICS

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Questions related to genetics covers topics including chromosomes, genes alleles, phenotypes, and genotypes. Some questions may also test your knowledge of the use of Punnett squares.

WHAT ARE GENETICS EXACTLY?!

Genetics is the discipline in biology wherein scientists focus on heredity. Whereas biology deals with individuals and groups, genetics deals with the heredity information carried in DNA.

CHROMOSOMES AND GENES

The chromosome is a fundamental unit of genetics. Contained in the nucleus of plant and animal cells, a chromosome is a linear strand that caries the hereditary information of the individual. It is composed of DNA and related proteins. Humans, for example, have 46 chromosomes, 23 each from their mother (through the egg cell) and from the father (carried in the sperm). All organisms that procreate through sexual reproduction are diploid, meaning they carry two sets of chromosomes. The two strands, connected by a single centromere, are referred to as chromatids. The number of chromosomes differ between species, and there is no correlation between the number of chromosomes in a species and the number of genes.

The gene is a core unit of genetics. It contains the heredity information that, singularly or through a particular grouping of genes, leads to a characteristic. A characteristic, or trait, may be physical, like eye color or left-handedness. It can also be behavioral or psychological, such as a predisposition to addictive behaviors. Genes are located in a particular position on a chromosome. They are the loci of mutation, which leads to genetic variation. Mutations occur randomly or through environmental agency.




 

UNDERSTANDING ALLELES

An allele is a version of a gene. A gene can be composed of a pair of alleles that call for a different distinct trait. In diploid organisms there are two alleles, one from each other. The alleles meet at a locus. For any pairing of alleles we can talk about an individual’s genes being homozygous or heterozygous. If the alleles are identical, the gene is homozygous. The characteristic that the gene produces will be the same as in the two parents. When the allele from each parent is different, the gene is heterozygous. Some traits are dominant, and some are recessive. The dominant trait is the one that will be expressed in the offspring.

PHENOTYPES AND GENOTYPES

Genetic variation takes place at the cellular level, but it can be seen on the surface in individuals. We can view an individual’s hair color, height, and hear his or her tone of voice. These observable characteristics are known as phenotypes. Some traits, like blood type, are not observable to the naked eye, but they are measurable, so they fall under the phenotype rubric. This includes those traits selected through heredity and influenced by environment. An individual’s height, for instance, is a product of its genes, but growing up in a poor environment with a lack of nutrition can lead to a stunting of growth. The observable height of the adult individual is part of its phenotype.

The genotype of an individual refers to the genetic makeup in its chromosomes. An individual may carry a recessive gene for a trait that does not appear in its phenotype but is still present and can be handed down to its genetic heirs. Although an individual’s height may be inhibited by its environment, its heirs still carry the genotype for a range of height. This is not to be confused with a genome. A genome refers to the entire genetic material of an individual. In a human’s case, all 46 chromosomes together. Genotype can refer to a specific allele. We can talk about an individual’s genotype carrying DNA for both blue and brown eyes. At the phenotype level, the individual has brown eyes because the trait is dominant.

UNDERSTANDING MENDEL

Many have called Gregor Mendel, a German monk who lived in the middle of the nineteenth century, the father of genetics. At the time, evolution was largely thought to work along Lamarckian lines, with traits being influenced by the environment. Mendel, through careful observation and experimentation, proved that heredity was instead at work. On the individual level, traits depended solely on the genes of the mother and the father. His work with pea plants led him to notice a mathematical distribution in traits among offspring allowing him to codify the laws of inheritance.

He discovered that the same traits are recessive, while others are dominant. He showed this by crossbreeding pea plants that varied in certain characteristics from height, color, and seed shape. By tracking appearance of phenotypic traits in the offspring, he developed a set of predictions now known as Mendel’s Laws of Heredity.




 

PUNNETT SQUARES

The English geneticist Reginald Punnett created a diagram for predicting the outcomes when crossbreeding genotypes. The Punnett square is used to show how the genes of parents (the genes of which are already known) might combine in their offspring. It is a simple box of four squares. The alleles of one parent are placed along the top, and the alleles of the other are placed along the side The four boxes show all the possible distribution of alleles in offspring (first filial generation or F1) of the two homozygous parents (parental generation, or P).

H H
h Hh Hh
h Hh Hh

H = Tall (dominant); h = Short (recessive)

In the Punnett square above, there is a 100% chance that the offspring will exhibit the dominant gene, becoming a tall pea plant.

In the following Punnett square, between two heterozygous parents with one dominant and one recessive allele, the offspring are shown to have 25% chance of exhibiting the recessive gene.

H h
H HH Hh
h Hh hh

H = Tall (dominant); h = Short (recessive)

When a homozygous parent with two recessive alleles and a parent with heterozygous alleles are crossed, the results are as shown below:

 

H H
h Hh Hh
h Hh Hh

H = Tall (dominant); h = Short (recessive)

Even though the recessive alleles represent 75% of the genetic material, the dominant gene will be represented in the phenotype 50% of the time. This demonstrates the difference between an organism’s genotype and phenotype.

MONOHYBRID INHERITANCE

Mendel’s First Law relates to monohybrid inheritance and is also known as the Law of Segregation. This law predicts the inheritance of a single trait. Punnett squares are useful tools for calculating the outcomes of crossbreeding.

In a typical monohybrid cross, we only record one trait. Let’s look at eye color. Both parents are homozygous, which means that they have both genes the same. One parent has the dominant trait and one has the recessive trait. In this case, brown eyes (B) are dominant and blue eyes (b) are recessive, so one parent is BB and one is bb. Since each parent denotes only one gene to the offspring, there is only one possible combination result for the first generation of offspring: one dominant and one recessive gene: Bb. This means that F1 generation is heterozygous, but the phenotype is the dominant brown eyes gene.

The first generation is then self-crossed produce the second generation, F2. For this generation, a Punnett square becomes useful for tracking the combinations. The parents are both Bb and the offspring are BB, Bb, Bb, and bb. The monohybrid ratio of the phenotypes here is 3:1.

B b
B BB Bb
b Bb bb

 

DIHYBRID INHERITANCE

Mendel’s Second Law relates to dihybrid inheritance and is also known as the Law of Independent Assortment. This law predicts the simultaneous inheritance of two separate and independent traits. Let’s look at eye color (brown or blue) and thumb shape (straight or curved). The parent generation begins with one parent homozygous and dominant for both traits and one parent homozygous recessive for both traits. We will use BBTT for brown eyes and straight thumbs and bbtt for blue eyes and curved thumbs. The F1 generation will inherit BT from one parent and bt from the other to get BbTt, which will result in brown eyes and straight thumbs. Gametes from the F1 generation can have BT, Bt, bT, or bt genotypes. When the F1 generation is self-crossed, we get the Punnett square shown below.

BT Bt bT bt
BT BBTT BBTt BbTT BbTt
Bt BBTt BBtt BbTt Bbtt
bT BbTT BbTt bbTT bbTt
bt BbTt Bbtt bbTt bbtt

 

Looking at the Punnett square, we can count 9 outcomes in which both B and T are dominant. There are 3 outcomes with B dominant and t recessive. There are 3 outcomes with b recessive and T dominant. There is only 1 outcome with both b and t recessive. This means a dihybrid cross has a 9:3:3:1 phenotype ratio.

There are also non-Mendelian inheritances that do not follow the 3:1 and 9:3:3:1 ratios due to co-dominance, incomplete dominant-recessive relationships, multiple alleles; or epistasis.

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