Reproduction and genetics, Year 11: genetic crosses and inherited disorders

Alleles

HintMost people carry two of these for each gene, one from each parent.

The whyA gene controls a characteristic, and it can exist in different forms. Each form is called an allele. An organism has two alleles of each gene, one inherited from each parent, and the pair may be the same or different.

dominant

HintIt is the allele that shows up whenever it is present.

The whyA dominant allele is always expressed, even when only one copy is present. The brown-fur allele here is recessive: it is expressed only when both copies are the same, with no dominant allele present.

Heterozygous

HintThe prefix is the Greek for 'other', as opposed to 'same'.

The whyIf the two alleles of a gene are different, as in Bb, the organism is heterozygous for that gene. If they are the same, as in BB or bb, it is homozygous.

Tt — the alleles it has

HintOne fact is what you could see; the other is what is written in its DNA.

The whyThe genotype is the alleles present. The phenotype is the characteristic that those alleles produce, here being tall. Two plants can share a phenotype and have different genotypes: TT and Tt are both tall.

Both must be the recessive allele

HintA single copy is not enough, because the other kind would mask it.

The whyA recessive allele is expressed only if two copies are present, which means that no dominant allele is there. An organism showing a recessive characteristic must therefore be homozygous for it.

1/4

HintOnly one of the four boxes in the Punnett square holds two small letters.

The whyEach parent's gametes carry T or t. The four equally likely combinations are TT, Tt, Tt and tt. Only tt is short, so the probability is 1 out of 4.

1 : 1

HintOne parent can only pass on b, so look at what the other parent adds.

The whyThe Bb parent's gametes carry B or b; every gamete from the bb parent carries b. The four boxes of the Punnett square are Bb, Bb, bb and bb: two black to two brown, which simplifies to 1 : 1.

30

HintShare the total into four equal parts.

The whyA ratio of 3 : 1 has 3 + 1 = 4 parts. One part is 120 ÷ 4 = 30, so 30 are expected to be short and 90 tall. Real results rarely match exactly, because a cross gives probabilities, not certainties.

Bb

HintThe baby needed a small letter from each of them, yet each of them shows the dominant colour.

The whyA brown mouse must be bb, so it received a b from each parent. Each parent is black, so each also has a B. Both parents are therefore heterozygous, Bb.

1/2

HintFirst work out the one genotype that every plant in the first set of offspring must have.

The whyThe RR parent gives only R and the rr parent only r, so every plant in the first set is Rr. Crossing Rr with Rr gives RR, Rr, Rr and rr. Two of the four are heterozygous, which is 1/2.

Polydactyly

Hint'Poly' means many; the rest comes from the Greek for finger.

The whyPolydactyly is caused by a dominant allele, so a person needs to inherit only one copy, from one parent, to have it. A parent who has one copy has a 1 in 2 chance of passing it to each child.

Its allele is dominant, so one copy is always expressed

HintCarriers exist only for conditions that need two copies to show.

The whyA carrier has one copy of an allele for a disorder but does not have the disorder. That can only happen when the allele is recessive and is masked by a dominant one. A dominant allele is never masked.

50

HintFill in the four boxes and count those holding one of each letter.

The whyEach parent's gametes carry F or f. The four equally likely combinations are FF, Ff, Ff and ff. Two of the four are Ff, so the chance of a carrier is 2 in 4, or 50%. The chance of a child with cystic fibrosis (ff) is 1 in 4.

Both parents are carriers: each has one recessive allele and passed it on

HintMum and Dad are healthy, yet something is masked in the two of them.

The whyCystic fibrosis, a disorder of cell membranes, is caused by a recessive allele, so a person has it only with two copies. A parent with one copy is healthy, because the dominant allele masks it, but can still pass the recessive allele to a child.

For: it can prevent the suffering caused by a serious disorder. Concern: embryos that are not chosen may be destroyed (others include the cost, and where the choosing of characteristics should stop).

HintOne side looks at a child's future health; the opposite side asks what happens to those left unselected.

The whyGCSE asks you to weigh economic, social and ethical points, using the information you are given. Economic points include the cost of screening set against the cost of lifelong treatment. Social and ethical points include reducing suffering, the fate of unused embryos, and how people living with a disorder may feel they are regarded.

XY

HintA female has two of the same; a male has one of each.

The whyOne of the 23 pairs of human chromosomes carries the genes that determine sex. Females have two X chromosomes and males have one X and one Y.

22

HintTake away the single pair that differs between males and females.

The whyOrdinary human body cells have 23 pairs of chromosomes. One pair carries the genes that determine sex, which leaves 23 − 1 = 22 pairs that control other characteristics only.

The father's: a sperm carries either an X or a Y, while every egg carries an X

HintAsk which gamete can come in two different kinds.

The whyA female is XX, so all her eggs carry an X. A male is XY, so half his sperm carry an X and half carry a Y. An X sperm gives XX, a girl; a Y sperm gives XY, a boy.

1/2 (50%)

HintEach fertilisation is a fresh event with no memory of earlier ones.

The whyThe cross XX × XY gives XX, XY, XX and XY: two girls to two boys, a ratio of 1 : 1. This is the chance at every fertilisation, whatever the sexes of the children born before.

X

HintA female's body cells hold two that are alike.

The whyFemales are XX. Meiosis puts one chromosome from each pair into each gamete, so every egg receives an X. Sperm come from XY cells, so they carry either an X or a Y.