bio cycle 5 genetics and inheritance

what a gene?

a gene is a segment of DNA that codes for a functional product, typically a protein

what are alleles?

different versions of the same gene that occupy the same locus on homologous chromosomes

what is a genotype?

the combination of alleles a person has for a particular gene

what is a phenotype?

the observable traits or characteristics produced by a genotype

what are homologous chromosomes?

chromosome pairs that have the same gene loci but may carry different alleles; one paternal and one maternal

why are alleles always in the same location on homologous chromosomes?

gene loci are fixed; crossing over in prophase I can exchange alleles between homologues, but the position of the gene does not change

what is homozygosity and heterozygosity?

• homozygous: two identical alleles (AA or aa)

• heterozygous: two different alleles (Aa)

what is a dominant allele?

an allele expressed with only one copy present; written as a capital letter

what is a recessive allele?

an allele expressed only when two copies are present, written as a lowercase letter

what was the blending theory of inheritance?

a false 1800s theory proposing that offspring are an average blend of parental phenotypes; cannot explain traits like blue eyes appearing from brown eyed parents

what made Mendel’s work significant?

• controlled crosses of pea plants

• quantitative data collection

• focused on discrete traits

• used ratios to infer inheritance rules

• applied scientific method rigorously

what are the components of Mendel’s explanatory model?

1. traits vary due to alleles

2. each organism inherits two alleles per trait

3. alleles segregate randomly into gametes

4. dominant alleles mask recessive ones in heterozygotes

what is the Law of Segregation?

alleles separate into different gametes during meiosis, giving each gamete a 50% chance of receiving either allele

what is independent assortment in a dihybrid?

alleles of different genes assort independently into gametes, producing 4 types of gametes from a heterozygous parent

what is a monohybrid cross?

a cross analyzing inheritance of one gene

what are typical monohybrid genotype and phenotype ratios?

• genotype: 1:2:1 (AA:Aa:aa)

• phenotype: 3:1 (dominant: recessive)

what is a dihybrid cross?

a cross examining two genes simultaneously

how many unique gametes can a parent produce?

2n, where n = number of heterozygous gene pairs

what is the expected phenotypic ratio of a dihybrid cross (AaBb x AaBb)?

9:3:3:1

• 9/16: dominant for both traits (AABB, AABb, AaBB, AaBb)

• 3/16: dominant for first trait, recessive for the second (AAbb, Aabb)

• 3/16: recessive for the first trait, dominant for the second (aaBB, aaBb)

• 1/16: recessive for both traits (aabb)

what is incomplete dominance?

heterozygotes exhibit an intermediate phenotype (ex: red flower RR x white flower rr = pink flower Rr)

what is codominance?

both alleles are fully expressed in heterozygotes (ex: red flower RR x white flower rr = red and white flower Rr, AB blood type)

what is a polygenic trait?

a trait influenced by many genes, producing continuous variation (ex: skin colour)

• many genes contribute small additive effects, produing a gradient of phenotypes

what is epistasis?

when one gene affects or masks the expression of another, altering Mendelian ratios

how does epistasis change expected ratios?

phenotypic and genotypic ratios deviate from 9:3:3:1 depending on gene interactions

what are sex-linked traits?

traits controlled by genes on sex chromosomes, usually X-linked

why are males more affected by X-linked recessive disorders?

males have only one X chromosome, so one mutated allele causes disease

why can’t fathers pass X-linked recessive traits to sons?

fathers pass on a Y chromosome to sons, not an X

what are common X-linked recessive disorders?

colour blindness, hemophilia, muscular dystrophy

how is eye colour inherited?

controlled by interaction of HERC2 and OCA2:

• HERC2 activates OCA2

• OCA2 increases melanin production

• more melanin = darker eyes

what type of inheritance explains blood types?

codominance (IA and IB) with a recessive allele (i)

what genotypes correspond to each blood type?

• type A: IAIA or IAi

• type B: IBIB or IBi

• type AB: IAIB

• type O: ii

why are blood transfusions type-specific?

mismatched surface antigens cause immune reactions