Inheritance and Mendel's law

What is an allele?

A different form of the same gene.

What does homozygous mean?

Having two identical alleles for a gene (e.g. AA or aa).

What does heterozygous mean?

Having two different alleles for a gene (e.g. Aa).

What is a dominant allele?

An allele that is always expressed, even if only one copy is present.

What is a recessive allele?

An allele that is only expressed if two copies are present.

What is a genotype?

The genetic makeup of an organism (the alleles it has).

What is a phenotype?

The physical expression of the genotype (the characteristics).

What is a Punnett square used for

To predict the probability of different genotypes and phenotypes in offspring.

Give two examples of genetic disorders.

• Cystic fibrosis → caused by a recessive allele.

• Polydactyly (extra fingers/toes) → caused by a dominant allele.

What chromosomes do females have?

XX.

What chromosomes do males have?

XY

What is the chance of having a male or female baby?

50% each.

Who was Gregor Mendel?

A monk who studied inheritance in pea plants in the 19th century.

What did Mendel discover?

Characteristics are determined by “hereditary units” (now called genes), which are passed unchanged from parents to offspring.

Why was Mendel’s work not accepted at first?

He had no knowledge of DNA or chromosomes, and scientists at the time didn’t understand his mathematical approach.

How has Mendel’s work been developed?

His “hereditary units” are now understood as genes carried on chromosomes, with different forms called alleles.

What does sex-linked mean?

A gene carried on the X chromosome (since the Y is smaller).

Give an example of a sex-linked disorder

Colour blindness or haemophilia (recessive alleles on the X chromosome).

What is genetic screening?

Testing embryos or adults for alleles that may cause genetic disorders.

What are the pros of genetic screening?

Reduces suffering, prevents costly treatment, helps parents make informed choices.

What are the cons of genetic screening?

Ethical issues (unused embryos in IVF), discrimination, “designer babies” concerns.