Module 7- Inheritance Flashcards
Define gene
Sequence of DNA nucleotide bases
Define allele
Altnernative form of a gene
Define recessive allele
The allele determines a characteristic only if no dominant allele is present
Characteristic is only expressed in phenotype when homozygous
Define dihybrid
Inheritance of 2 genes
Define monohybrid
Inheritance of 1 gene
Define dominant allele
This allele determines the development of a characteristic, even if one copy is present
A single allele is required for a charcteristic to be expressed in phenotype
Define a homozygous chromosome
A pair of chromosomes that have the same alleles of a gene
Define a heterozygous chromosome
A pair of chromosomes that have different alleles of a gene
Define gametes
An egg cell or a sperm cell
Define genotype
The alleles present in an organism
Define phenotype
The physical appearance of an individual determined by the genotype and environment
Define multiple gametes
A gene that has many different forms e.g. blood group
Define codominance
When both alleles are expressed equally
2 marks
Males are more likely than females to show a phenotype produced by a recessive allele carried on the X chromosome. Explain why.
Sex-linked genes
- Males have only one allele
- Females need two recessive alleles
2 marks
Explain what is meant by the term phenotype
1.Characteristic due to genetic genotype /alleles
2.Characteristic due to environment
2 marks
The actual % of heterozygous is likely to be lower in future generations. Explain why
- Affected individuals usually do not reproduce
- Die during childhood
- Do not pass on allele
- Genetic screening
1 mark
In fruit flies, the genes for body colour and wing length are linked. Explain what this means
Genes on the same chromosome
2 marks
In fruit flies, males have the sex chromosomes XY and the females have XX. In fruit flies, a gene for eye colour is carried on the X chromosome. The allele for red eyes, R, is dominant to the allele for white eyes, r.
Male fruit flies are more likely than female fruit flies to have white eyes. Explain why.
- Males have only one allele
- Females need two recessive alleles
3 marks
In fruit flies, the genes for body colour and for wing development are not on the sex chromosomes. The allele for grey body colour, G, is dominant to the allele for black body colour, g. The allele for long wings, L, is dominant to the allele for short wings, l.
A geneticist carried out a cross between fruit flies with grey bodies and long wings (heterozygous for both genes) and fruit flies with black bodies and short wings.
Grey body and long wings- 223
Black body and short wings- 218
Explain the results.
- The 2 genes are linked (autosomal linkage)
- No cossing over (crossing over less likely to occcur if linked genes are close together)
- No Gl and no gL gametes produced
2 marks
In genetic crosses, the observed phenotypic ratios obtained in the offspring are often not the same as the expected ratios.
Suggest two reasons why.
- Small sample size
- Random fertilisation
- Linked Genes
- Epistasis
- Lethal genotypes
A student investigated the monohybrid inheritance of eye shape in fruit flies. Two fruit flies with bar (narrow) eyes were crossed. Of the offspring, 1538 had bar eyes and 462 had round (normal) eyes.
Using suitable symbols, give the genotypes of the parents. Explain your answer.
- Bb / suitable equivalent
- Both parents have bar eyes, but have some offspring with round eyes, so parents must be carriers of recessive allele for round eyes
Define genes locus
Location of a gene on a chromosome
2 marks
Explain how the observation of chromosomes from an embryo cell could enable the sex to be determined
- Female are XX, males are XY
- Y chromosome is smaller than X
2 marks
During meiosis, when the X and Y chromosomes pair up, they do not form a typical bivalent as do other chromosomes, Explain why
- X and Y chromosome are different sizes/ Y very short
- Only short ‘pairing region’
