UNIT 2 AOS1 revision

What is a gene?

A segment of DNA that codes for a specific protein or functional RNA.

What is an allele?

A different form or variation of the same gene.

What is the genome?

The complete set of genetic material in an organism.

What is the difference between a gene and an allele?

A gene is a DNA sequence coding for a trait; alleles are alternative versions of that gene that may produce different phenotypes.

What is a locus?

The specific location of a gene on a chromosome.

What are homologous chromosomes?

A pair of chromosomes (one from each parent) with the same genes at the same loci but possibly different alleles.

What is the difference between autosomes and sex chromosomes?

Autosomes are non-sex chromosomes; sex chromosomes determine biological sex (e.g., X and Y in humans).

How many chromosomes do humans have?

46 chromosomes (23 pairs); diploid = 46, haploid = 23.

How do chromosome size and number vary in different organisms?

Different species have different chromosome counts and sizes; e.g., dogs have 78 chromosomes, fruit flies have 8.

What is a karyotype?

A visual arrangement of all chromosomes in a cell, used to detect abnormalities.

Give two examples of abnormalities detectable by karyotyping.

Trisomy 21 (Down syndrome) and Turner syndrome (XO).

What is the purpose of meiosis?

To produce haploid gametes for sexual reproduction, ensuring genetic diversity.

How is meiosis different from mitosis?

Meiosis produces four genetically unique haploid cells; mitosis produces two identical diploid cells.

What happens during the diploid → haploid process?

Two rounds of cell division reduce chromosome number by half.

What happens during Meiosis I?

Homologous chromosomes separate, producing two haploid cells with duplicated chromosomes.

What happens during Meiosis II?

Sister chromatids separate, producing four unique haploid cells.

What is crossing over?

Exchange of genetic material between homologous chromatids during prophase I.

Why is crossing over significant?

It creates new allele combinations, increasing genetic diversity.

What is independent assortment?

Random orientation of homologous chromosome pairs during metaphase I, leading to varied gametes.

What are linked genes?

Genes located on the same chromosome that tend to be inherited together.

What is recombination frequency?

The percentage of offspring showing new allele combinations due to crossing over.

What is a map unit?

A unit representing 1% recombination frequency, used to estimate gene distances.

What is a genotype?

The genetic makeup of an organism for a trait.

What is a phenotype?

The observable characteristics of an organism, resulting from genotype and environment.

How does a dominant allele affect phenotype?

It masks the effect of a recessive allele when heterozygous.

How does a recessive allele affect phenotype?

It is expressed only when two copies are present.

What is codominance? Give an example.

Both alleles are fully expressed; e.g., AB blood type.

What is incomplete dominance? Give an example.

The heterozygous phenotype is intermediate; e.g., red × white flowers producing pink.

What notation is used for dominant/recessive alleles?

Capital letter for dominant (A), lowercase for recessive (a).

What notation is used for codominant alleles?

Different capital letters or symbols; e.g., IA, IB, i.

How are sex-linked alleles written?

With the allele as a superscript on the X or Y chromosome; e.g., Xᴴ, Xʰ.

What is a polygenic trait? Give an example.

A trait influenced by multiple genes; e.g., human height.

What is a monogenic trait? Give an example.

A trait controlled by a single gene; e.g., cystic fibrosis.

How can environment influence phenotype? Give an example.

Sun exposure affecting skin pigmentation.

How can epigenetics influence phenotype? Give an example.

DNA methylation silencing a gene; e.g., stress altering gene expression.

What is a monohybrid cross?

A cross examining the inheritance of a single gene.

What are the genotype and phenotype ratios for Aa × Aa?

Genotype: 1 AA : 2 Aa : 1 aa; Phenotype: 3 dominant : 1 recessive.

What is a test cross?

Crossing an individual with an unknown genotype with a homozygous recessive individual.

What is a dihybrid cross?

A cross examining two genes simultaneously.

What is the phenotype ratio for AaBb × AaBb with independent assortment?

9:3:3:1.

How do you predict outcomes for linked genes?

Use parental and recombinant gametes, considering recombination frequency.

How are linked genes written?

E.g., AB/ab for both genes on one chromosome.

How do you calculate recombination frequency?

(Number of recombinants ÷ total offspring) × 100.

What does a 1% recombination frequency mean?

The genes are 1 map unit apart.

What does a square represent in a pedigree?

Male.

What does a circle represent in a pedigree?

Female.

How is an affected individual shown in a pedigree?

Shaded symbol.

How is a carrier shown in a pedigree?

Half-shaded or with a dot in the middle.

How is mating shown in a pedigree?

Horizontal line between two symbols.

How is offspring shown in a pedigree?

Vertical line down from parents.

Pattern for autosomal dominant inheritance?

Appears in every generation; equal in males and females.

Pattern for autosomal recessive inheritance?

May skip generations; equal in males and females.

Pattern for X-linked dominant inheritance?

Affected fathers pass trait to all daughters, no sons.

Pattern for X-linked recessive inheritance?

More males affected; passed from mother to son.

How to tell autosomal vs. sex-linked in a pedigree?

Sex-linked often affects one sex more; autosomal affects sexes equally.

How to tell dominant vs. recessive in a pedigree?

Dominant traits appear in every generation; recessive traits may skip generations.

What is the founder effect?

Reduced genetic variation due to a small founding population.

What is genetic drift?

Random changes in allele frequencies in small populations.

How can Punnett squares be applied to real-world case studies?

To predict inheritance risk for diseases or traits in families.

What is the role of pedigree analysis in genetic counselling?

To determine inheritance patterns and assess risk.

Name one ethical issue in human genetic testing.

Privacy concerns, discrimination, or consent.

How can environmental and genetic factors combine to influence a trait?

Genetic predisposition plus environmental trigger causes the phenotype.