Flashcards in 6.2 - Patterns of Inheritance Deck (57)
How does sexual reproduction lead to variation in genotypes in species?
Meiosis makes gametes with a unique assortment of alleles through crossing-over & independent assortment of chromosomes. Also random fusion of gametes during fertilisation also increases variation in off-spring.
Explain the meaning of the term crossing over.
Swapping of alleles between maternal & paternal homologous chromosomes during prophase I of meiosis. Pairs of homologous sister chromatids exchange portions resulting in a shuffling of alleles.
Other than inheriting characteristics, what else may have an effect on the phenotype of an individual?
What is genotype?
The genetic constitution of an organism.
What is phenotype?
Appearance due to alleles. Appearance due to environment.
What is a dominant allele?
A allele that is always shown in the phenotype.
What is a recessive allele?
An allele that is only shown in the phenotype when homozygous.
What is heterozygous?
1 dominate allele, 1 recessive allele.
What is homozygous dominant?
2 dominant alleles.
What is homozygous recessive?
2 recessive alleles.
What is co-dominance?
Where both alleles are dominant, so both are expressed in the phenotype.
What is multiple alleles?
Where there are more than 2 alleles of a particular gene.
How can you prove that the allele for a genetic characteristic is on the X chromosome?
Male offspring are positive for the phenotype even when the father is negative and the mother is negative (heterozygous) because faulty X can only be inherited from mother.
How is a genetic diagram laid out? – to explain flower colour being controlled by a single gene with two codominant alleles.
Parental phenotypes: red flower X white flower
Parental genotypes: CR CR X CW CW
Gametes: CR CW
F1 genotypes: All CRCW
F1 phenotypes: All pink
Gametes: CR, CW, CR, CW
F2 genotypes: CRCR ; CRCW ; CRCW ; CWCW
F2 phenotypes: red ; pink (pink) ; white
Expected F2 phenotypic ratio: 1 :2: 1
Why is a recessive genetic disorder, found on the X chromosome, more likely to shown on male’s phenotypes than female’s?
Males only have one X chromosome so only one allele is needed for it to be expressed. Females need two alleles for it to show in their phenotype.
What is monohybrid inheritance?
The inheritance of a characteristics controlled by a single gene.
What is dihybrid inheritance?
The inheritance of 2 characteristics which are controlled by different genes.
What are the predicted phenotypic ratios for monohybrid, dihybrid and codominant crosses?
Monohybrid cross - 3:1 - dominant : recessive
Dihybrid cross - 9:3:3:1 - dominant both: dominant 1st recessive 2nd: recessive 1st dominant 2nd: recessive both
Codominant - 1:2:1 - homozygous for one allele: heterozygous: homozygous for the other allele
Why are the ratios of offspring genotypes not the same in real life to those calculated in genetic diagrams?
Fertilisation is random.
Why can the allele frequency of a lethal genetic disorder decrease over time?
Affected individuals might struggle to reproduce, therefore the allele will not be passed on to future generations.
What is an autosome?
Any chromosome that is not a sex chromosome (X or Y).
Explain the term linkage.
Two or more genes (loci) on the same chromosome. They do not independently assort during meiosis (metaphase I). Their alleles are passed on to offspring together.
How and when can linked genes be split up?
Crossing over during meiosis (prophase I) can split linked genes.
When are linked genes less likely to be split up?
The closer together two genes are on the autosome, the more closely they are said to be linked and the less likely they are to be split up by crossing over.
Describe and explain the predicted ratio of two autosomally linked alleles.
Predicted ratio is the same as a monohybrid cross = 3:1 as the two autosomally linked alleles are inherited together. A higher proportion of the offspring will have parental genotypes and phenotypes.
What is epistasis?
One gene masks the expression of another gene.
What is recessive epistasis?
Having 2 copies of the recessive epistatic allele blocks the expression of the other gene.
What is the predicted ratio of a dihybrid cross involving a recessive epistatic allele?
What is dominant epistasis?
Having one copy of the dominant epistatic allele blocks the expression of the other gene.