Codominance and sex linkage
Introduced| English | Meaning |
|---|---|
| codominance/ˈkəʊdəmɪnəns/ | Inheritance where both alleles are fully expressed in the heterozygote, as in AB blood group. |
| sex linkage/seks ˈlɪŋkɪdʒ/ | sex linkage |
| sex chromosomes/seks ˈkrəʊməsəʊmz/ | Chromosomes involved in sex determination; the usual human school model uses XX and XY. |
| colour blindness/ˈkʌlə ˈblaɪndnəs/ | Reduced ability to distinguish some colours; the common red-green forms are usually inherited through an X-linked recessive allele. |
When inheritance is more complex
- Not every trait is a simple dominant-or-recessive. Some alleles share the spotlight, and some traits depend on your sex.
- These are codominance 共显性 and sex linkage 伴性遗传.
In codominance:
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Both alleles are expressed (e.g. blood group AB).
A person with both A and B blood-group alleles has blood group ______.
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Codominance gives group AB.
Codominance
- In codominance, both alleles show their effect in a heterozygote.
- Example: in blood groups, a person with both A and B alleles has blood group AB.
XX x XY gives a 1:1 ratio of girls to boys; the sperm decides the sex
Codominance cross
When both alleles show in the phenotype (codominance), the heterozygote shows BOTH features — work it out with a Punnett square.
The sex chromosomes of a human male are:
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Males are XY; females are XX.
Whose gamete determines the sex of a baby?
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The mother always gives X; the father gives X or Y.
Sex chromosomes 性染色体
- Sex is determined by the sex chromosomes: XX (female) and XY (male).
- The father's gamete (X or Y) determines the baby's sex.
Recessive sex-linked conditions affect females more often than males.
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False. Males have only one X chromosome, so a single recessive allele is expressed; females need two copies. These conditions affect males more often.
Sex linkage
- A sex-linked gene is carried on the X chromosome.
- Because males have only one X, they are more likely to show recessive sex-linked conditions (like red-green colour blindness 色盲).
Sex-linked recessive conditions affect more males. A male has only one X chromosome, so a single recessive allele on it will show. A female would need it on both X chromosomes.
You've got it
- codominance: both alleles show (e.g. blood group AB)
- sex chromosomes: XX = female, XY = male; the father determines sex
- sex-linked genes are on the X — recessive ones affect males more often
Write from an XY parent
- One parent is XX.
- The other is XY and can pass on X or Y.
Who determines the sex of a child, and what is special about blood group AB?
The sperm supplies the chromosome that differs.
Which parent's gamete decides whether a child is XX or XY?
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Only the father can pass a Y.