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

Alleles

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

为什么A 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

提示It is the allele that shows up whenever it is present.

为什么A 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

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

为什么If 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

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

为什么The 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

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

为什么A 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

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

为什么Each 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

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

为什么The 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

提示Share the total into four equal parts.

为什么A 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

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

为什么A 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

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

为什么The 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

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

为什么Polydactyly 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

提示Carriers exist only for conditions that need two copies to show.

为什么A 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

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

为什么Each 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

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

为什么Cystic 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).

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

为什么GCSE 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

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

为什么One 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

提示Take away the single pair that differs between males and females.

为什么Ordinary 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

提示Ask which gamete can come in two different kinds.

为什么A 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%)

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

为什么The 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

提示A female's body cells hold two that are alike.

为什么Females 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.