1.8: Introduction to heredity review

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Key terms

Term	Meaning
Genetics	The study of biological inheritance
Trait	A specific characteristic of an individual
Gene	A unit of heredity that is passed from parent to offspring
Allele	One of different forms of a gene
Genotype	The genetic makeup of an organism (ex: TT)
Phenotype	The physical characteristics of an organism (ex: tall)
Dominant allele	Allele that is phenotypically expressed over another allele
Recessive allele	Allele that is only expressed in absence of a dominant allele
Homozygous	Having two identical alleles for a particular gene
Heterozygous	Having two different alleles for a particular gene
Punnett square	Diagram that can be used to predict the genotypes and phenotypes resulting from a genetic cross

Mendelian inheritance

Gregor Mendel's principles of heredity, observed through patterns of inheritance in pea plants, form the basis of modern genetics.

Mendel proposed that traits were specified by "heritable elements" called genes. Genes come in different versions, or alleles, with dominant alleles being expressed over recessive alleles. Recessive alleles are only expressed when no dominant allele is present.

In most sexually reproducing organisms, each individual has two alleles for each gene (one from each parent). This pair of alleles is called a genotype and determines the organism's appearance, or phenotype.

Mendel's laws

Table showing how genes exchange according to segregation or independent assortment during meiosis and how this translates into the Mendel's Laws. — Laws of segregation and independent assortment. Image modified from Wikimedia, Public domain

When an organism makes gametes, each gamete receives just one gene copy, which is selected randomly. This is known as the law of segregation.

Mendel's second law is the law of independent assortment, which states that the alleles for one gene sort into gametes independently of the alleles of another gene.

Punnett squares and probability

A Punnett square can be used to predict genotype and phenotypes of offspring from genetic crosses. A single-gene, or monohybrid cross is pictured below.

This illustration shows a monohybrid cross. In the P generation, one parent has a dominant yellow phenotype and the genotype YY, and the other parent has the recessive green phenotype and the genotype yy. Each parent produces one kind of gamete, resulting in an F_{1} generation with a dominant yellow phenotype and the genotype Yy. Self-pollination of the F_{1} generation results in an F_{2} generation with a 3 to 1 ratio of yellow to green peas. One out of three of the yellow pea plants has a dominant genotype of YY, and 2 out of 3 has the heterozygous genotype Yy. The homozygous recessive plant has the green phenotype and the genotype yy. — Monohybrid Punnett square. Image modified from OpenStax, CC BY 4.0

A test cross can be used to determine whether an organism with a dominant phenotype is homozygous or heterozygous.

In a test cross, a parent with a dominant phenotype but unknown genotype is crossed with a recessive parent. If the parent with the unknown genotype is homozygous dominant, all the resulting offspring will have at least one dominant allele. If the parent with the unknown genotype is heterozygous, 50 percent of the offspring will inherit a recessive allele from both parents and will have the recessive phenotype. — Example test cross. Image credit: OpenStax, CC BY 4.0

Punnett squares can be used for two-gene crosses, or dihybrid crosses by following the same basic rules as for a monohybrid cross. However, since there are now more gamete types, there must also be more squares in the table.

Illustration of the hypothesis that the seed color and seed shape genes assort independently. In this diagram, the Y and R alleles of the yellow, round parent and the y and r alleles of the green, wrinkled parent are not inherited as units. Instead, the alleles of the two genes are inherited as independent units. P generation: A yellow, round plant (YYRR) is crossed with a green, wrinkled plant (yyrr). Each parental generation can produce only one type of gamete, YR or yr. F1 generation: The F1 dihybrid seeds are yellow and round, with a genotype of YyRr. The F1 plants can produce four different types of gametes: YR, Yr, yR, and yr. We can predict the genotypes of the F2 plants by placing these gametes along the top and side axes of a 4X4 Punnett square and filling in the boxes to represent fertilization events. F2 generation: Completion of the Punnett square predicts four different phenotypic classes of offspring, yellow/round, yellow/wrinkled, green/round, and green/wrinkled, in a ratio of 9:3:3:1. This is the prediction of the model in which the seed shape and seed color genes assort independently. Punnett square:||YR|Yr|yR|yr-|-|-|-|-|-YR||YYRR|YYRr|YyRR|YyRrYr||YYRr|_YYrr_|YyRr|_Yyrr_yR||YyRR|YyRr|**yyRR**|**yyRr**yr||YyRr|_Yyrr_|**yyRr**|***yyrr*** Plain text = yellow, round phenotype _Italic text_ = yellow, wrinkled phenotype **Bold text** = green, round phenotype ***Bold, italic text*** = green, wrinkled phenotype — Dihybrid cross. Image credit: "OpenStax," CC BY 4.0.

Probabilities in genetics

The two probability rules that are most relevant to Punnett squares are the product rule and the sum rule.

The product rule states that the probability of two (or more) independent events occurring together can be calculated by multiplying the individual probabilities of the events.

Illustration of how a Punnett square can represent the product rule. Punnett square:||A|a-|-|-|-A||AA|**Aa**a||_Aa_|***aa*** There's a 1/2 chance of getting an a allele from the male parent, corresponding to the rightmost column of the Punnett square. Similarly, there's a 1/2 chance of getting an a allele from the maternal parent, corresponding to the bottommost row of the Punnett square. The intersect of these the row and column, corresponding to the bottom right box of the table, represents the probability of getting an a allele from the maternal parent and the paternal parent (1 out of 4 boxes in the Punnett square, or a 1/4 chance). — Example of the product rule using a Punnett square.

In some genetics problems, you may need to calculate the probability that any one of several events will occur. In this case, you’ll need to apply another rule of probability, the sum rule. According to the sum rule, the probability that any of several mutually exclusive events will occur is equal to the sum of the events’ individual probabilities.

Illustration of how a Punnett square can represent the sum rule. Punnett square:||A|a-|-|-|-A||**AA**|**Aa**a||**Aa**|aa The **bolded** boxes represent events that result in a dominant phenotype (AA or AA genotype). In one, an A sperm combines with an A egg. In another, an A sperm combines with an a egg, and in a third, an a sperm combines with an A egg. Each event has a 1/4 chance of happening (1 out of 4 boxes in the Punnett square). The chance that any of these three events will occur is 1/4+1/4+1/4 = 3/4. — Example of the sum rule using a Punnett square.

Common mistakes and misconceptions

Dominant traits are not always the most common. Some people may think that dominant trait is the most likely to be found in the population, but the term "dominant" only refers to the fact that the allele is expressed over another allele. An example of this is Huntington's disease. Even though Huntington's is caused by a dominant allele, it only affects about 30,000 people in the United States¹.
Traits are not always the product of a single gene. For example, there are at least 3 different genes that are associated with eye color in humans. In addition, there are sometimes more than two alleles for each gene. For example, there are 3 different alleles of one gene that determine coat color of cats.

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Khan Academy (CC BY-NC-SA 3.0; All Khan Academy content is available for free at www.khanacademy.org)

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