Chapter 12

In a cross of Aa Bb cc X Aa Bb Cc, what is the probability of obtaining an individual with the genotype AA Bb Cc?

1/16

Japanese four o'clocks that are red and tall are crossed to white short ones, producing an F1 that is pink and tall. If these genes assort independently, and the F1 is self-crossed, what would you predict for the ratio of F2 phenotypes?

3 red tall:6 pink tall:3 white tall:1 red short:2 pink short:1 white short

In pleiotropy, a single gene can affect more than one trait.

A pleiotropic effect occurs when an allele affects more than one trait. These effects are difficult to predict.

Some human traits exhibit dominant/recessive inheritance.

Certain human traits have been found to have a Mendelian basis. Inheritance patterns in human families can be analyzed and inferred using a pedigree diagram of earlier generations.

Dihybrid cross probabilities are based on monohybrid cross probabilities.

Dihybrid cross probabilities are based on monohybrid cross probabilities.

Phenotypes may be affected by the environment.

Genotype determines phenotype, but the environment will have an effect on this relationship. Environment means both external and internal factors. For example, in Siamese cats, a temperature-sensitive enzyme produces more pigment in the colder peripheral areas of the body.

When you cross true-breeding tall and short tobacco plants you get an F1 that is intermediate in height. When this F1 is self-crossed, it yields an F2 with a continuous distribution of heights. What is the best explanation for these data?

Height is determined by the additive effects of many genes.

Which of the following is NOT a part of Mendel's five-element model?

If an allele is present it will be expressed.

Traits in a dihybrid cross behave independently.

If parents differing in two traits are crossed, the F1 will be all dominant. Each F1 parent can produce four different gametes that can be combined to produce 16 possible outcomes in the F2. This yields a phenotypic ratio of 9:3:3:1 of the four possible phenotypes.

The Testcross: Revealing Unknown Genotypes

In a testcross, an unknown genotype is crossed with a homozygous recessive genotype. The F1 offspring will all be the same if the unknown genotype is homozygous dominant. The F1 offspring will exhibit a 1:1 dominant:recessive ratio if the unknown genotype is heterozygous.

Dominance is not always complete.

In incomplete dominance the heterozygote exhibits an intermediate phenotype; the monohybrid genotypic and phenotypic ratios are the same. Codominant alleles each contribute to the phenotype of a heterozygote.

In polygenic inheritance, more than one gene can affect a single trait.

Many traits, such as human height, are due to multiple additive contributions by many genes, resulting in continuous variation.

The F1 generation exhibits only one of two traits, without blending.

Mendel called the trait visible in the F1 the dominant trait; the other he termed recessive.

What property distinguished Mendel's investigation from previous studies?

Mendel quantified his results.

The 3:1 ratio is actually 1:2:1.

Mendel then examined the F2 and found the recessive F2 plants always bred true, but only one out of three dominant F2 bred true. This means the 3:1 ratio is actually 1 true-breeding dominant:2 non-true-breeding dominant:1 recessive.

Mendel used mathematics to analyze his crosses.

Mendel's experiments involved reciprocal crosses between true-breeding pea varieties followed by one or more generations of self-fertilization. His mathematical analysis of experimental results led to the present model of inheritance.

Early plant biologists produced hybrids and saw puzzling results.

Plant breeders noticed that some forms of a trait can disappear in one generation only to reappear later, that is, they segregate rather than blend.

Mendel's Principle of Independent Assortment explains dihybrid results.

The Principle of Independent Assortment states that different traits segregate independently of one another. The physical basis of independent assortment is the independent behavior of different pairs of homologous chromosomes during meiosis I.

Two probability rules help predict monohybrid cross results.

The rule of addition states that the probability of two independent events occurring is the sum of their individual probabilities. The rule of multiplication, or product rule, states that the probability of two independent events both occurring is the product of their individual probabilities.

Genes may have more than two alleles.

There may be more than two alleles of a gene in a population. Given the possible number of DNA sequences, this is not surprising.

Many traits result from the additive expression of more than one gene.

This is called polygenic inheritance.

Mendel's Principle of Segregation explains monohybrid observations.

Traits are determined by discrete factors we now call genes. These exist in alternative forms we call alleles. Individuals carrying two identical alleles for a gene are said to be homozygous, and individuals carrying different alleles are said to be heterozygous. The genotype is the entire set of alleles of all genes possessed by an individual. The phenotype is the individual's appearance due to these alleles. The Principle of Segregation states that during gamete formation, the two alleles of a gene separate (segregate). Parental alleles then randomly come together to form the diploid zygote. The physical basis of segregation is the separation of homologues during anaphase of meiosis I.

The F2 generation exhibits both traits in a 3:1 ratio.

When F1 plants are self-fertilized, the F2 shows a consistent ratio of 3 dominant:1 recessive. We call this 3:1 ratio the Mendelian monohybrid ratio.

The F1 generation of the monohybrid cross purple (PP) × white (pp) flower pea plants should

all have purple flowers.

An organism's________ is/are determined by its______________

alleles; phenotype

Phenotypes like height in humans, which show a continuous distribution, are usually the result of

the action of multiple genes on a single phenotype.

The F1 plants from the previous question are allowed to self-fertilize. The phenotypic ratio for the F2 should be

3 purple:1 white.

In epistasis, interactions of genes alter genetic ratios.

Genes encoding enzymes that act in a single biochemical pathway are not independent. In corn, anthocyanin pigment production requires the action of two enzymes. Doubly heterozygous individuals for these enzymes yield a 9:7 ratio when self-crossed

The Punnett square allows symbolic analysis.

Punnett squares are formed by placing the gametes from one parent along the top of the square with the gametes from the other parent along the side. Zygotes formed from gamete combinations form the blocks of the square

If the two genes in the previous question showed complete linkage, what would you predict for an F2 phenotypic ratio?

1 red tall:2 pink tall:1 white short

Mendel's model assumes that each trait is determined by a single factor with alternate forms. We now know that this is too simplistic and that

a single gene can affect more than one trait, and traits may be affected by more than one gene.

What is the probability of obtaining an individual with the genotype bb from a cross between two individuals with the genotype Bb?

¼