Genetics Decoded

Genetics Decoded

Why do you have your grandmother's eyes? Inheritance follows rules you can actually **predict**: with a little genetic vocabulary and a simple grid. Learn the language of alleles, run a cross yourself, read a family tree, and work out the odds of inheriting a trait.

The language of inheritance

A **gene** can come in different versions, and for each gene you inherit **two** of them, one from each parent. Seven words carry the whole topic:

Say it properly

A pea plant has the alleles Bb. Because its two alleles are different, this plant is _____. The letters Bb are its _____, while the flower colour you can actually see is its _____. A plant with bb has two of the same allele, and because both copies are recessive it is the only genotype that shows the _____ characteristic.

Same flowers, same alleles?

Two pea plants both have purple flowers, and B is the dominant allele for purple. Can you work out their genotypes just by looking at them?

• No: both BB and Bb produce purple flowers, so the phenotype does not reveal the genotype
• Yes: a BB plant has darker flowers than a Bb plant
• Yes: only BB plants can have purple flowers
• No, because genotype and phenotype mean the same thing

The Punnett square ▦

To predict a cross, draw a **Punnett square**. Put one parent's two gametes across the **top** and the other parent's down the **side**. Each cell combines a top allele with a side allele to give an offspring **genotype**. Two conventions the examiners expect: - Write each genotype with the **dominant (capital) allele first**, so **Bb**, never **bB**. - **Show the gametes**, and always **state the ratio or probability** at the end. A filled grid with no conclusion underneath it leaves marks on the table.

Cross them: Bb × Bb

Both parents are Bb. Fill in the four offspring genotypes.

• BB
• Bb
• bb

Work out the odds

From that Bb × Bb cross, what is the probability (as a percentage) that an offspring is homozygous recessive (bb)?

The phenotype ratio

In that Bb × Bb cross (B dominant), what is the ratio of dominant to recessive **phenotype** in the offspring?

• 3 : 1
• 1 : 1
• 1 : 3
• 1 : 2 : 1

Two inherited disorders

Some disorders are caused by a single faulty allele. The specification names one of each kind on purpose, because they behave in opposite ways in a family:

Two healthy parents

Two parents who do not have cystic fibrosis can have a child who does. Two parents who do not have polydactyly cannot have a child who does. Why the difference?

• Cystic fibrosis is recessive, so each unaffected parent can carry one hidden copy. Polydactyly is dominant, so an unaffected parent carries no copy at all
• Cystic fibrosis is simply more common than polydactyly
• Polydactyly can only be inherited from the father
• Cystic fibrosis is caused by two genes and polydactyly by one

Read the family tree

Squares are male, circles are female, and a shaded shape means the person has cystic fibrosis, which is recessive. Individual 4 is affected. Tap the TWO people who must both be carriers.

How sex is decided

Of your 23 pairs of chromosomes, the 23rd pair are the **sex chromosomes**: - **XX** = female - **XY** = male The mother is XX, so **every** egg carries an X. The father is XY, so each sperm carries **either an X or a Y**. Whichever sperm arrives first supplies the second chromosome, and that is what decides the sex.

Cross for sex: XX × XY

Mother XX (down the side), father XY (across the top). Fill in the offspring.

• XX
• XY

Whose gamete decides?

Explain why it is the father's gamete, and not the mother's, that determines the sex of a child.

• Say which sex chromosomes the mother has, and what every egg therefore carries
• Say which the father has, and what each sperm can carry
• Say which gamete supplies the deciding chromosome, and why that follows
• State the expected ratio of females to males, and where in the cross it comes from

In the exam

The alleles an organism carries are its _____; the characteristic they produce is its phenotype, and a dominant phenotype can hide a recessive allele behind it. In a Bb × Bb cross the phenotype ratio is _____ dominant to 1 recessive. Polydactyly is caused by a _____ allele and cystic fibrosis by a recessive one, which is why only cystic fibrosis has unaffected _____.