Mandelian Genetics (Punnent Square) Flashcards

In cats, which trait is dominant: long hair or short hair?

Short hair.

What is the genotype of a true-breeding short-haired cat?

Homozygous dominant (SS).

What is the genotype of a long-haired cat?

Homozygous recessive (ss)

Crossing SS (short hair) × ss (long hair) produces what offspring phenotype?

All short-haired kittens.

If two cats produce both short- and long-haired kittens, what must the short-haired parent’s genotype be?

Heterozygous (Ss).

In humans, which allele determines the ability to taste PTC?

Dominant allele (T).

Genotype of a nontaster for PTC?

Homozygous recessive (tt).

If two heterozygous tasters (Tt × Tt) mate, what fraction of children are tasters?

3/4 tasters.

In the same cross (Tt × Tt), what fraction of children are nontasters?

1/4 nontasters.

What is the probability the first child of Tt × Tt parents will be a taster?

75%.

What is the probability the first three children of Tt × Tt parents are all nontasters?

(1/4)³ = 1/64.

In flowers, blue (B) is dominant over white (b). What is the phenotype of bb?

White flowers.

Crossing Bb (blue) × bb (white) produces what phenotypic ratio?

1 blue : 1 white.

If all offspring are blue from a B_ × bb cross, what is the blue parent’s genotype?

Homozygous dominant (BB).

A heterozygous blue flower (Bb) produces which gametes?

B and b.

If two Bb blue flowers are crossed, what genotypic ratio results?

1 BB : 2 Bb : 1 bb.

What phenotypic ratio results from Bb × Bb in flowers?

3 blue : 1 white.

Widow’s peak is dominant. Can two parents with widow’s peaks have a child without one?

Yes.

Which genotype explains a widow’s peak parent producing a child without it?

Heterozygous (Ww).

Attached earlobes are recessive. If Mr. Jones has attached lobes and half his children do too, what is his genotype?

Homozygous recessive (ee).

Mrs. Jones has free earlobes and half her children have attached lobes. What is her genotype?

Heterozygous (Ee).

Curly hair shows incomplete dominance. What is the heterozygote’s phenotype?

Wavy hair.

If two curly-haired (CC) parents had a wavy-haired child, what does this suggest?

At least one parent was heterozygous (CS).

What is the expected ratio from crossing two wavy-haired parents (CS × CS)?

1 curly : 2 wavy : 1 straight.

Why does Tay Sachs disease persist in a population despite being fatal before reproduction?

Carriers (heterozygotes) are unaffected and maintain the allele.

If 80% of people can taste PTC, which allele is dominant?

Tasting allele.

In fruit flies, which genotype has normal wings?

cy cy.

What phenotype does Cy cy (heterozygote) produce in fruit flies?

Curly wings.

What happens to flies with Cy Cy genotype?

Lethal, do not hatch.

In a Cy cy × Cy cy cross, what fraction of eggs survive?

3/4 survive.

From the same cross, what fraction of surviving offspring have straight wings (cy cy)?

1/3.

From the same cross, what fraction of surviving offspring have curly wings (Cy cy)?

2/3.

If a white-eyed fly crossed with a red-eyed fly produces only red-eyed offspring, which allele is dominant?

Red eye allele.

Genotype of a white-eyed fly with white-eyed parents?

Homozygous recessive.

Genotype(s) of the red-eyed offspring from a red × white cross?

Heterozygous

If a heterozygous red-eyed offspring is crossed with a white-eyed parent, what is the expected ratio?

1 red : 1 white.

Crossing two heterozygous red-eyed flies (Rr × Rr) yields what genotypic ratio?

1 RR : 2 Rr : 1 rr.

In the same cross (Rr × Rr), what phenotypic ratio results?

3 red : 1 white.

What does “true-breeding” mean in genetics?

Homozygous genotype.

What is a Punnett square used for?

Predicting offspring genotypes and phenotypes.

What is the genotype ratio from Ss × Ss in cats?

1 SS : 2 Ss : 1 ss

What is the phenotype ratio from Ss × Ss in cats?

3 short hair : 1 long hair.

If both parents are heterozygous for a trait, what is the probability of producing a homozygous recessive child?

25%.

In a monohybrid cross, what is the expected genotypic ratio?

1 : 2 : 1.

In a monohybrid cross, what is the expected phenotypic ratio?

3 : 1.

A heterozygote is sometimes called what?

A carrier.

What does incomplete dominance mean?

Heterozygote shows an intermediate phenotype.

What does codominance mean?

Both alleles are expressed equally in the phenotype.

Widow’s peak (W) is dominant. What is the genotype of someone without a widow’s peak?

ww.

If both parents have widow’s peaks but a child doesn’t, what must both parents be?

Heterozygous (Ww).

If Mr. Smith accuses Mrs. Smith of infidelity because their child lacks a widow’s peak, is he justified?

No.

What does the presence of recessive traits in offspring tell us about parents’ genotypes?

Both parents carry at least one recessive allele.

Free earlobes are dominant (E). What is the genotype of a child with attached earlobes?

ee.

If 3 out of 6 Jones children have attached earlobes, what is the probability per child?

50%.

Incomplete dominance: what ratio results from CS × CS in hair texture?

1 curly : 2 wavy : 1 straight.

Why is incomplete dominance different from standard Mendelian dominance?

The heterozygote has a distinct phenotype.

What explains why actual offspring numbers may differ from predicted ratios?

Chance variation / statistical probability.

What is a lethal allele?

An allele that causes death when homozygous.

Example of a lethal allele in fruit flies?

Cy (curly wing) in homozygous condition.

From a Cy cy × Cy cy cross, what fraction of eggs do not hatch?

25%.

From the same cross, what fraction of hatched flies are curly winged?

2/3.

Why do recessive lethal alleles persist in populations?

Carriers are unaffected and pass on the allele.

In the eye-color problem, what evidence proves red eyes are dominant?

Red-eyed offspring from a red × white cross.

If two heterozygous red-eyed flies mate, what percentage of offspring are expected to have white eyes?

25%.

What is the probability that two heterozygous tasters (Tt × Tt) produce 4 tasters in a row?

(3/4)⁴ = 81/256.

Why doesn’t Tay Sachs disappear despite being fatal?

Hidden in heterozygous carriers.

What term describes a recessive allele that persists due to carrier frequency?

Balanced polymorphism.

If 20% of people cannot taste PTC, what is their genotype?

tt.

What proportion of gametes from a Bb parent carry the recessive allele?

50%.

In a Bb × Bb cross, what is the probability of producing a homozygous dominant (BB) offspring?

25%.