Basic Principles of Heredity

Who is known as the father of genetics?

Gregor Mendel.

Who was Gregor Johann Mendel?

An Augustinian priest and teacher who bred pea plants from 1856–1863.

When did Mendel publish his work, and when was it recognized?

He published it in 1866; it had little impact at the time and was not recognized until it was reproduced in 1900.

What experimental subject did Mendel use?

The pea plant, Pisum sativum.

Gene

An inherited factor that helps determine a characteristic.

Allele

A variant or alternative form of a gene.

Locus

A specific place on a chromosome occupied by an allele.

Genotype

A set of alleles possessed by an organism.

Homozygote

An organism possessing two of the same alleles at a locus.

Heterozygote

An organism possessing two different alleles at a locus.

Characteristic

An attribute or feature possessed by an organism.

Phenotype

The appearance or manifestation of a characteristic.

Monohybrid cross

A cross between parents that differ in a single characteristic.

P (parental) generation

The first generation of a cross.

F1 (first filial) generation

Offspring of the P generation.

Reciprocal cross

A cross in which the phenotypes of the male and female parents are reversed.

F2 (second filial) generation

Offspring of the F1 generation through self-fertilization.

Backcross

A cross between an F1 offspring and either parental genotype.

What must offspring inherit, even if they phenotypically display only one factor?

Genetic factors from both parents.

What happens to alleles when gametes form?

They separate, with one allele going into each gamete.

How do two alleles enter gametes?

They separate into the gametes with equal probability.

Dominant allele or phenotype

Expressed in homozygotes and heterozygotes; denoted by a capital letter.

Recessive allele or phenotype

Expressed only in homozygotes; denoted by a lowercase letter.

Principle of segregation

A diploid organism possesses two alleles at a locus, and the alleles separate when gametes form, with one going to each gamete.

Concept of dominance

When two different alleles occur in a genotype, only the dominant allele is observed in the phenotype.

What is a Punnett square?

A shorthand method for determining the outcome of a genetic cross by combining gametes from parental generations.

Who developed the Punnett square, and when?

English geneticist Reginald C. Punnett in 1917.

Probability

The likelihood that an event occurs; it ranges from 0 to 1.

How can probability be calculated?

The number of times an event takes place divided by all possible outcomes.

Multiplication rule

The probability of two or more independent events taking place together is calculated by multiplying their probabilities.

What keyword signals the multiplication rule?

AND.

What is the probability of rolling a 4 twice?

1/6 × 1/6 = 1/36.

Addition rule

The probability of two or more mutually exclusive events is calculated by adding the probabilities of the events.

What keyword signals the addition rule?

OR.

What is the probability of rolling a 3 or 4?

1/6 + 1/6 = 2/6 = 1/3.

For a Tt × Tt cross, what alleles do the gametes contain?

Half have the T allele and half have the t allele.

For a Tt × Tt cross, what is the probability of tall progeny?

1/4 + 1/4 + 1/4 = 3/4.

Conditional probability

Probability modified by additional information.

What progeny are expected from a Tt × Tt cross?

1/4 TT, 1/2 Tt, and 1/4 tt.

How does a tall phenotype modify the possible genotypes in a Tt × Tt cross?

Tall plants can be TT or Tt, never tt.

What is the ratio of TT to Tt among tall progeny from a Tt × Tt cross?

1:2.

What is the probability that a tall progeny from a Tt × Tt cross is heterozygous Tt?

2/3.

What inheritance pattern does albinism have in the example?

Recessive.

For Aa × Aa, what are the probabilities of normal pigmentation and albinism?

3/4 normal pigmentation and 1/4 albino.

For Aa × Aa parents, what is the probability that all three progeny have albinism?

1/64.

For Aa × Aa parents, what is the probability of one child with albinism and two without?

27/64.

In the binomial expression, what does p represent?

The probability of one event, such as a child having albinism.

In the binomial expression, what does q represent?

The probability of the alternative event, such as a child not having albinism.

In the binomial expression, what does n represent?

The number of times the event occurs.

What binomial expansion is used for five children?

(p + q)^5 = p^5 + 5p^4q + 10p^3q^2 + 10p^2q^3 + 5pq^4 + q^5.

What does each term in a binomial expansion give?

The probability for one particular combination.

What does p^5 represent in the five-child albinism example?

Five children with albinism.

What does 5p^4q represent in the five-child albinism example?

Four children with albinism and one without.

What term gives the probability of two children with albinism and three without?

10p^2q^3.

What is the probability of two children with albinism in the five-child example?

10(1/4)^2(3/4)^3 = 270/1024 = 0.26.

Test cross

A genetic cross in which an individual of unknown genotype is crossed with an individual with a homozygous recessive genotype.

In a test cross of an unknown tall pea plant, what does all tall progeny indicate?

The unknown plant is TT.

In a test cross of an unknown tall pea plant, what do 1/2 tall and 1/2 short progeny indicate?

The unknown plant is Tt.

Wild type

The most common allele for a characteristic in animals; it is often symbolized by letter(s) and a plus sign (+).

How are letters for genetic symbols usually chosen?

They are usually based on the less common phenotype.

What does ye represent in the oriental fruit fly?

Yellow eyes.

What does ye+ represent in the oriental fruit fly?

Wild-type eye color.

Genetic heterogeneity

A phenomenon in which the same phenotype can result from genetic variation at one of several loci; example: albinism.

Multifactorial inheritance

Alleles at multiple loci combine to influence a phenotype, and environmental factors frequently play a role; example: type 2 diabetes.

Genetic-environmental interaction

The effect of a gene depends on the environment in which it occurs; example: depression.

Dihybrid cross

A cross between two individuals who differ in two characteristics, such as seed shape and seed color.

Principle of independent assortment

Alleles at different loci separate independently of one another.

When does independent assortment not apply?

When the loci are on the same chromosome, unless crossing over occurs.

What can cause observed ratios to differ from expected ratios?

Chance.

Chi-square goodness-of-fit test

A statistical test used to evaluate how well a set of observed values fit expected values.

What does the chi-square goodness-of-fit test not tell you?

Whether a genetic cross was carried out correctly, whether the results are right, or whether the correct genetic explanation was chosen.

What does the chi-square goodness-of-fit test indicate?

The probability that the difference between observed and expected values is due to chance.

In cats, how do black and gray coat-color alleles relate?

Black coat color (B) is dominant to gray coat color (b).

How are degrees of freedom calculated?

n − 1, where n is the number of different phenotypes.

What cutoff do most scientists use to accept that chance may be responsible for a deviation?

0.05.

What does a probability below 0.05 indicate in this test?

Chance is not responsible and a significant difference exists.

What does a significant difference mean?

A factor other than chance contributed to the deviation between observed and expected values.