Genetics · Chapter 6: Sex, pedigrees and sex-linked traits

Year 11 Science · About 15 minutes with activities · Turn your sound on

Chapter 6 · Pedigrees & sex-linked traits

Boysabout 1 in 12

Girlsabout 1 in 200

22 other pairs

+ 1 pair of sex chromosomes

Female: XXtwo of the same

Male: XYtwo different

Mother: XX

Father: XY

Every egg carries X

Half of sperm X, half Y

1 female : 1 male50% : 50%

Every baby: 50% chanceeach fertilisation is independent

Square = male

Circle = female

Same colour = same phenotypecoloured = has the trait

Breeding pairhorizontal line

Offspringvertical lines down

3 generationseach on its own level

Recessive patternunaffected parents → affected child

Dominant patternaffected parents → unaffected child

X^bY: affectedone recessive allele is enough

X^BX^b: carriernormal phenotype

Carrier mother × normal father½ of sons affected · ½ of daughters carriers

Chapter 6 recap

Sex chromosomes: females XX, males XY. Every egg carries X; half of sperm carry X and half carry Y, so the sperm determines sex. Fertilisation is random and independent: always a 50% chance of a boy.
Pedigree charts: squares = males, circles = females. Unaffected parents with an affected child → recessive (parents heterozygous). Affected parents with an unaffected child → dominant.
Sex-linked genes are on a sex chromosome. Males (X^bY) need only one recessive allele to be affected; females need two (X^bX^b), otherwise they are carriers (X^BX^b). Examples: haemophilia, red-green colour blindness.
Next: Chapter 7 · Biotechnology and DNA sequencing

Why are more boys colour blind?

Chapter 6 · About 15 minutes · sound on

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