Final Exam Review Part 2- Mendelian Genetics and Punnett squares

In sheep, belly fur (A) is dominant to no belly fur (a). Two heterozygous sheep (Aa × Aa) mate. What fraction of offspring will have belly fur?

• 1/4
• 1/2
• 3/4
• 4/4

Answer explanation

Aa × Aa gives AA (1/4), Aa (2/4), aa (1/4). AA and Aa both have belly fur = 3/4.

Tags

NGSS.HS-LS3-3

In sheep, belly fur (A) is dominant to no belly fur (a). Two heterozygous sheep (Aa × Aa) mate. Which of the following genotypes will appear in the offspring? Select ALL that apply.

• AA
• Aa
• aa
• aA is different from Aa

Answer explanation

Aa × Aa produces AA (25%), Aa (50%), and aa (25%). aA and Aa are the same genotype.

Tags

NGSS.HS-LS3-1

NGSS.HS-LS3-2

What is the key difference between complete dominance and incomplete dominance?

• Complete dominance produces 4 phenotypes; incomplete dominance produces 2
• In complete dominance, the dominant allele fully masks the recessive; in incomplete dominance, the heterozygote shows a blended intermediate phenotype
• Incomplete dominance only occurs in plants
• Complete dominance only applies to X-linked genes

Answer explanation

Complete dominance: one allele fully masks the other. Incomplete dominance: heterozygote shows a mix (e.g., red × white = pink).

A red-flowered plant (RR) is crossed with a white-flowered plant (rr). The offspring are all pink. What type of inheritance does this demonstrate?

• Complete dominance
• Codominance
• Incomplete dominance
• Multiple alleles

Answer explanation

Pink is a blend of red and white — neither allele fully dominates. This is incomplete dominance.

Red flower allele (R) and white flower allele (W) are codominant. What will the heterozygous (RW) flower look like?

• Entirely red
• Entirely white
• Pink (a blend of red and white)
• Red and white patches or spots

Answer explanation

Codominance means both alleles are fully expressed simultaneously — the flower shows distinct red AND white patches, not a blend.

How can you distinguish codominance from incomplete dominance in a heterozygous individual?

• You cannot — they produce identical phenotypes
• Incomplete dominance produces a blended intermediate phenotype; codominance shows both original phenotypes simultaneously
• Codominance always involves X-linked genes
• Incomplete dominance only affects animals

Answer explanation

Key distinction: incomplete dominance = blend (pink); codominance = both expressed at once (red AND white patches).

What does it mean for a gene to have 'multiple alleles'?

• An individual organism carries more than 2 alleles for the gene
• More than 2 versions of the allele exist in a population, though any one individual still carries at most 2
• The gene is located on multiple chromosomes
• Multiple genes all code for the same protein

Answer explanation

Multiple alleles = more than 2 versions exist in the population. Any diploid individual still carries only 2 alleles (one per homolog).

Tags

NGSS.HS-LS3-3

Which of the following is the best example of a gene with multiple alleles?

• Eye color, controlled by a single gene with only 2 versions
• Height, controlled by many different genes
• ABO blood type, controlled by a single gene with I^A, I^B, and i^o alleles

• Skin color, controlled by several genes each with 2 alleles

Answer explanation

ABO blood type has three alleles (I^A, I^B, i) for a single gene — a classic example of multiple alleles.

What is polygenic inheritance?

• One gene controls multiple traits simultaneously
• Multiple genes each contribute to a single trait, producing a range of phenotypes
• A trait controlled by alleles on the X chromosome
• When heterozygotes show a blend of both phenotypes

Answer explanation

Polygenic inheritance = multiple genes influence one trait. This produces a spectrum of phenotypes rather than just dominant or recessive.

A student asks why human height isn't simply 'tall' or 'short' like pea plant height in Mendel's experiments. What is the best explanation?

• Human height is X-linked, so it differs between males and females
• Height is determined by incomplete dominance in humans
• Human height is polygenic — many genes each contribute to it, creating a wide range of heights in a population
• Mendel's laws do not apply to humans at all

Answer explanation

Human height is polygenic — many genes contribute small effects, resulting in a continuous spectrum of heights rather than 2-3 discrete categories.

Tags

NGSS.HS-LS3-3