Mendel Genetics MCQ practice

The phenotypic ratio for the F1 generation that results from a test cross between a homozygous recessive individual and a heterozygous individual is

1:1

What type of cross is used to determine if an individual with the dominant form of a trait is homozygous or heterozygous?

A test cross

What type of cross is used to determine if an individual with the dominant form of a trait is homozygous or heterozygous?

A test cross

A male fruit fly has the genotype PpYYrrTt. In terms of these 4 allele pairs, how many different types of gametes can he form?

The male fruit fly can form 4 different types of gametes

• Formula: 2^n (where n is the number of heterozygous allele pairs)

• Genotype: PpYYrrTt

• Heterozygous pairs (n): Pp and Tt (2 heterozygous pairs)

• Homozygous pairs: YY and rr (no variation)

• Math: 2^n = 2² = 4 different gamete combinations (PYrT, PYrt, pYrT, pYrt)

A male fruit fly has the genotype PpYYrrTt. In terms of these 4 allele pairs, how many different types of gametes can he form?

The male fruit fly can form 4 different types of gametes

• Formula: 2^n (where n is the number of heterozygous allele pairs)

• Genotype: PpYYrrTt

• Heterozygous pairs (n): Pp and Tt (2 heterozygous pairs)

• Homozygous pairs: YY and rr (no variation)

• Math: 2^n = 2² = 4 different gamete combinations (PYrT, PYrt, pYrT, pYrt)

Write the different types of gametes the male fruit fly can form:

The male fruit fly can form 4 different types of gametes

• PYrT

• PYrt

• pYrT

• pYrt

To find the number of different gametes, look at each allele pair:

• Pp is heterozygous (2 options: P or p)

• YY is homozygous (1 option: Y)

• rr is homozygous (1 option: r)

• Tt is heterozygous (2 options: T or t)

Multiply the number of possible alleles for each gene pair:

• Number of gametes = 2 × 1 × 1 × 2 = 4

Write the different types of gametes the male fruit fly can form:

The male fruit fly can form 4 different types of gametes

• PYrT

• PYrt

• pYrT

• pYrt

To find the number of different gametes, look at each allele pair:

• Pp is heterozygous (2 options: P or p)

• YY is homozygous (1 option: Y)

• rr is homozygous (1 option: r)

• Tt is heterozygous (2 options: T or t)

Multiply the number of possible alleles for each gene pair:

• Number of gametes = 2 × 1 × 1 × 2 = 4

If  R = red pigment and r = no pigment. In carnations, RR offspring make a lot of red pigment, rr offspring make no pigment and Rr offspring make a small amount of red pigment, thus appearing pink. Pink carnations are therefore an example of

Incomplete dominance

Achondroplasia is caused by an autosomal dominant mutation in the fibroblast growth factor receptor 3 (FGFR3) gene. A person who carries this mutation has shorter limbs than average. Two copies of the mutant gene are invariably fatal before or shortly after birth. If a person with achondroplasia has children with a person of average height, what is the probability that both their first child and second child will have achondroplasia?

The probability that both the first child and second child will have achondroplasia is 1/4 (or 25%)

• Parent with achondroplasia: Heterozygous (Aa), meaning they carry one mutant allele (A) and one normal allele (a)

• Parent of average height: Homozygous recessive (aa), meaning they carry two normal alleles

• Chance per pregnancy: When an (Aa) parent and an (aa) parent have a child, there is a 1/2 (50%) chance the child inherits the mutant A gene (achondroplasia) and a 1/2 (50%) chance they inherit the normal a gene (average height)

• Independent events: Because each pregnancy is an independent event, you multiply the probability of the first child having achondroplasia (1/2) by the probability of the second child having achondroplasia (1/2)

• ½ x ½ = ¼ or 25%

Incomplete dominance

A pattern of non-Mendelian inheritance where the heterozygote displays a blended, intermediate phenotype because neither allele is completely dominant over the other

• a non-Mendelian inheritance pattern in which neither allele is fully dominant, so heterozygotes show an intermediate phenotype that blends both alleles (like a red and white flower producing pink)

Codominance

A Non-Mendelian inheritance pattern where both alleles for a gene are fully and simultaneously expressed in the phenotype of a heterozygote, without blending or masking each other

• In a heterozygous individual, neither allele is dominant or recessive. Both alleles are independently transcribed and translated into functional protein products that appear side-by-side in the phenotype

• Codominance shows both traits distinctly (e.g., red and white spots or hairs).

• Incomplete dominance blends traits into an intermediate phenotype (e.g., red and white flowers making pink flowers)

• Individuals receive one version of a gene, called an allele, from each parent. If the two alleles are different, they will both be expressed equally in the phenotype.

Test cross

A genetic experimental method used to determine the unknown genotype of an individual that displays a dominant phenotype

• type of cross that is used to determine if an individual with the dominant form of a trait is homozygous or heterozygous

• because an organism expressing a dominant trait could be either homozygous dominant (AA) or heterozygous (Aa), it is crossed with an organism that is homozygous recessive (aa) for that specific trait. The phenotypes of the resulting offspring reveal the genetic identity of the mystery parent

Dihybrid cross

A genetic mating experiment between two organisms that are both heterozygous for two different traits (such as AaBb x AaBb)

• Tracks the simultaneous inheritance of two distinct genes located on different (non-homologous) chromosomes

• Parental Genotype: typically represented as AaBb x AaBb (double heterozygotes)

• Gamete Formation: each parent produces 4 possible gamete combinations using the FOIL method (First, Outside, Inside, Last). For example, AaBb produces AB, Ab, aB, and ab

• Punnett Square: uses a 4×4 grid (16 boxes) to map out all potential offspring combinations

• Classic Phenotypic Ratio: when both genes show complete dominance and assort independently, a cross between two double heterozygotes yields a 9:3:3:1 phenotypic ratio in the F2 generation

Dihybrid cross

A genetic mating experiment between two organisms that are both heterozygous for two different traits (such as AaBb x AaBb)

• Tracks the simultaneous inheritance of two distinct genes located on different (non-homologous) chromosomes

• Parental Genotype: typically represented as AaBb x AaBb (double heterozygotes)

• Gamete Formation: each parent produces 4 possible gamete combinations using the FOIL method (First, Outside, Inside, Last). For example, AaBb produces AB, Ab, aB, and ab

• Punnett Square: uses a 4×4 grid (16 boxes) to map out all potential offspring combinations

• Classic Phenotypic Ratio: when both genes show complete dominance and assort independently, a cross between two double heterozygotes yields a 9:3:3:1 phenotypic ratio in the F2 generation

Test cross

A type of cross that is used to determine if an individual with the dominant form of a trait is homozygous or heterozygous