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.

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

1. Guinea pigs Bb × Bb (black fur dominant over white)
2. Boxes: BB, Bb, Bb, bb
3. Black (BB or Bb): 3 of 4. White (bb): 1 of 4
4. Ratio black : white = 3 : 1
5. Probability of white = 1/4 = 0.25 = 25%
6. 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.

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.