Reproduction and the menstrual cycle (HL)

Sexual reproduction uses meiosis and fertilisation to create variation. In humans the menstrual cycle is coordinated by hormones:

• FSH stimulates follicle growth and oestrogen production;
• a mid-cycle LH surge triggers ovulation;
• progesterone (from the corpus luteum) maintains the uterus lining.

These hormones interact by positive and negative feedback.

Dihybrid crosses and linkage (HL)

A dihybrid cross of two genes (unlinked) between double heterozygotes (AaBb × AaBb) gives a 9:3:3:1 phenotype ratio.

Linked genes lie on the same chromosome and tend to be inherited together, unless separated by crossing over (recombination). Some traits are sex-linked (e.g. haemophilia on the X chromosome).

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Homeostasis and feedback

Homeostasis uses negative feedback to hold variables near a set point.

• Blood glucose: insulin lowers it, glucagon raises it.
• Water balance: ADH increases water reabsorption in the kidney when you are dehydrated, giving concentrated urine.
• Temperature and thyroxine also show feedback control.

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