Meiosis makes gametes (eggs and sperm). The DNA is copied, then the cell divides twice, making four genetically different cells, each with half the number of chromosomes (23 in humans). Fertilisation restores the full number, and the mixing of alleles creates variation.
Your genotype is the alleles you have, such as Bb; your phenotype is the feature you show, such as brown eyes. Two identical alleles (BB or bb) are homozygous; two different alleles (Bb) are heterozygous. A Punnett square predicts offspring. Crossing two heterozygous parents (Bb × Bb) gives a 3 : 1 ratio of dominant to recessive phenotypes. Sex is decided by chromosomes: XX is female and XY is male, so each child has a 50% chance of being either.
Worked example
Cystic fibrosis is recessive (f). Two carriers (Ff) have a child. What is the probability the child has CF?
- Each parent can pass on F or f.
- Punnett square: FF, Ff, Ff, ff.
Answer:One in four is ff, so the probability is 0.25 (25%).
Key words
- meiosis
- division that makes four different gametes
- genotype
- the alleles present, e.g. Bb
- phenotype
- the characteristic shown
- heterozygous
- two different alleles
- homozygous
- two identical alleles
New questions every time.
NGSS-referenced: HS-LS3-1