Genetic crosses, family trees and sex
Alleles, genotype and phenotype
A gene can come in different versions called alleles.
Body cells have two alleles of each gene, one from each parent.
- A dominant allele (capital letter, B) shows even if only one copy is present.
- A recessive allele (small letter, b) shows only if there are two copies.
- Homozygous: two identical alleles (BB or bb). Heterozygous: two different alleles (Bb).
- The genotype is the alleles (Bb). The phenotype is the characteristic you see (black fur).
Most characteristics are controlled by several genes working together; a few, like the ones in this lesson, are controlled by a single gene.
The Punnett square
Write the gametes of one parent along the top and the other down the side, then fill each box with one allele from each.
| B | b | |
|---|---|---|
| B | BB | Bb |
| b | Bb | bb |
- Guinea pigs Bb × Bb (black fur dominant over white)
- Boxes: BB, Bb, Bb, bb
- Black (BB or Bb): 3 of 4. White (bb): 1 of 4
- Ratio black : white = 3 : 1
- Probability of white = 1/4 = 0.25 = 25%
- Out of 40 young, expect 0.25 × 40 = 10 white
Each box is equally likely, so every box is one quarter (25%). Bb × bb gives 1 : 1, and BB × bb gives all black.
A 3 : 1 ratio means 3 out of 4, not 1 out of 3. Count the boxes, then divide by 4.
Family trees and sex
In a family tree, squares are males, circles are females, and a filled shape has the condition.
For a recessive condition, anyone with it is homozygous recessive (tt). An unaffected parent or child of an affected person must be a carrier (Tt). If the tree gives no clue, the person could be TT or Tt.
Human body cells have 23 pairs of chromosomes; one pair are the sex chromosomes. Females are XX and males are XY.
| X | X | |
|---|---|---|
| X | XX | XX |
| Y | XY | XY |
Every egg carries an X; half the sperm carry X and half carry Y. So each child has a 50% chance of being a boy, whatever the earlier children were. For two separate events, multiply: two girls in a row is ½ × ½ = ¼.
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