7.1 Inheritance Flashcards
(25 cards)
Write a definition for each of these key words:
Genotype The genetic constitution of an organism
Phenotype The expression of the genotype and its interaction with the environment
Allele A form of a gene
Dominant allele The allele of a gene that is expressed in the phenotype, even in the presence of an alternative allele
Recessive allele The allele of a gene that is only expressed in the phenotype if there is another identical allele present
Codominant allele Both alleles of a gene are expressed in the phenotype of a heterozygote
Homozygous When an individual contains two identical alleles of a gene
Heterozygous When an individual contains two different alleles of a gene
Write a definition for each of these key words:
Genotype The genetic constitution of an organism
Phenotype The expression of the genotype and its interaction with the environment
Allele A form of a gene
Dominant allele The allele of a gene that is expressed in the phenotype, even in the presence of an alternative allele
Recessive allele The allele of a gene that is only expressed in the phenotype if there is another identical allele present
Codominant allele Both alleles of a gene are expressed in the phenotype of a heterozygote
Homozygous When an individual contains two identical alleles of a gene
Heterozygous When an individual contains two different alleles of a gene
What are the key pieces of information you should always include when drawing out a genetic cross?
phenotype of parents
- genotype of parents
- What are the genotypes of the gametes each parent can produce
- punnet square – put the gametes on the outside of the square
- genotype of offspring
- phenotype of offspring
- Ratio of the phenotypes of the offspring
- What is a monohybrid cross?
A genetic cross which just shows one gene
- Complete a genetic cross for two heterozygous drosophila:
Parent phenotype Normal wings Normal wings
Parent genotype Ww Ww
Gametes W w W w
W w W WW Wild-type Ww Wild-type w Ww Wild-type Ww Vestigial
Offspring genotype WW Ww ww
Offspring phenotype Wild-type Wild-type vestigial
Ratio of phenotypes 3 Wild-type = 1 vestigial
- What would you expect the ratio of offspring phenotypes to be when two heterozygous parents breed?
3 dominant phenotype : 1 recessive phenotype
- How do you explain evidence from a pedigree chart to show that an individual is heterozygous?
- Individual (named) has the dominant phenotype (named) therefore must have dominant allele.
- But has offspring/parent with recessive phenotype (named) therefore must carry recessive allele
- Example of offspring with a different phenotype to BOTH parents
- Both parents must pass on an allele they are not expressing to that offspring
- As they are displaying another allele, they must be heterozygous
- Assuming the gene is not sex linked: What is the evidence you look for in a pedigree chart to confirm that an allele is recessive?
Find an example of an offspring with a different phenotype to BOTH parents. The parents must therefore be heterozygous, so whatever phenotype they express must be the dominant one. Use the answer structure below:
• Name the two parents who have the same phenotype as each other (the dominant phenotype)
• Name the offspring who has a different phenotype (the recessive phenotype)
• The parents must both be heterozygous because
o They have the dominant phenotype, so must have the dominant allele
o They pass on the recessive allele to their offspring
- Assuming the gene is not sex linked: What is the evidence you look for in a pedigree chart to confirm that an allele is dominant?
Same
- If the gene is sex linked what is the evidence you look for in a pedigree chart to confirm that an allele is recessive?
• Name the two parents who have the same phenotype as each other (the dominant phenotype)
• Name the offspring who has a different phenotype (the recessive phenotype)
• The MOTHER must be heterozygous because
o She has the dominant phenotype, so must have the dominant allele
o She passes on the recessive allele to her offspring
• (NOTE: The father cannot be heterozygous because he only has one copy of the X chromosome – so only has one copy of the allele)
- What are codominant alleles?
When both alleles are expressed in the phenotype
- What is meant by multiple-alleles?
When there are more than two possible alleles for a gene
- Blood type is controlled by multiple alleles. What are the three alleles which exist for blood type?
IA, IO and IB
Sex linkage
- What is ‘sex linkage’?
When a gene is found on the X or Y chromosome
- Complete these rules which are always true if a gene is found on the X-chromosome. And explain them.
A dad with the dominant phenotype will always have … daughters with the dominant phenotype
Because … the father only has one copy of the X-chromosome and it carries the dominant allele. His daughters will inherit the X-chromosome carrying this dominant allele.
A mother with the recessive phenotype will always have … sons with the recessive phenotype
Because … the mother must have two X chromosomes that both have the recessive allele. Her sons will inherit the Y chromosome from the dad and the X chromosome with the recessive allele from the mother.
- (a) What evidence should you look for to show that a gene is NOT found on the X-chromosome?
A dominant dad that has a recessive daughter
A recessive mum that has a dominant son
(b) How do you structure an explanation for this?
• State the relevant parent + offspring with their phenotype
• If the gene was on the X chromosome,
Parent’s genotype would be …
Offspring’s genotype would be …
• This is impossible because parent ‘…’ would have passed on the X chromosome with the …………… allele to his/her daughter/son
• So IF it was sex-linked, the offspring would have to have the dominant/recessive phenotype
- What is a dihybrid cross?
A genetic cross which shows two genes
- In a dihybrid cross you consider the inheritance of two different genes at the same time. E.g. gene A and gene B
What is the genotype of a body cell of an individual that is heterozygous for both gene A and B?
AaBb
- Draw the arrangement of these genes on chromosomes assuming that there is no autosomal linkage:
- Draw the 4 possible gametes which could be formed from this body cell:
- Draw the 2 possible ways that these alleles could be arranged on chromosomes in a body cell if there is autosomal linkage.
For each cell draw the two types of gametes which are most likely to form and the two types of gametes which will only form if there is crossing over
Possible body cells
Most common gametes
Gametes which will only form if there is crossing over
Example: In drosophila ‘wild-type’ wings are dominant to ‘vestigial’ wings
Wild-type = W vestigial = w
Red eyes are dominant to purple eyes
Red eyes = R purple eyes = p
- Draw the arrangement of these genes on chromosomes assuming that there is no autosomal linkage:
- Draw the 4 possible gametes which could be formed from this body cell:
- Draw the 2 possible ways that these alleles could be arranged on chromosomes in a body cell if there is autosomal linkage.
For each cell draw the two types of gametes which are most likely to form and the two types of gametes which will only form if there is crossing over
Possible body cells
Most common gametes
Gametes which will only form if there is crossing over
Example: In drosophila ‘wild-type’ wings are dominant to ‘vestigial’ wings
Wild-type = W vestigial = w
Red eyes are dominant to purple eyes
Red eyes = R purple eyes = p
- Complete a genetic cross for two drosophila which are heterozygous for both characteristics. Assume that the genes are not linked
Parent phenotype Wild-type and red eyed Wild-type and red eyed Parent genotype WwRr WwRr Gametes WR Wr wR wr WR Wr wR wr WR Wr wR wr WR WWRR Wild-type Red eyed WWRr Wild-type Red eye WwRR Wild-type Red eye WwRr Wild-type Red eye Wr WWrR Wild-type Red eye WWrr Wild-type Purple eye WwrR Wild-type Red eye Wwrr Wild-type Purple eye wR wWRR Wild-type Red eye wWRr Wild-type Red eye wwRR Vestigial Red eye wwRr Vestigial Red eye wr wWrR Wild-type Red eye wWrr Wild-type Purple eye wwrR Vestigial Red eye wwrr vestigial purple eye
Offspring phenotype Wild type Red eyed Wild type Purple eyed Vestigial Red eyed vestigial Purple eyed Phenotype Ratio 9 3 3 1
- What would you expect the ratio to be for the offspring phenotypes for a genetic cross between two parents which are both heterozygous for two genes?
9 : 3 : 3: 1
- How can you identify if there is autosomal linkage occurring? Explain your answer
- Use chi squared to check if the ratio of phenotypes in the offspring is significantly different to what you would expect (i.e. in this case the ratio will not be 9 : 3 : 3 : 1)
- This is because some of the alleles are linked.
- Therefore, it is more likely that some gametes will be formed than others.
- Therefore some of the offspring phenotypes will occur more or less often than you would have expected
- What is epistasis?
When genes interact with each other.
example fur colour is controlled by several different genes.
C = the fur can have colour c = there is no colour (albino)
A = agouti = band of yellow on an otherwise dark hair a = no band of yellow
B = black fur b = brown fur
What would the phenotype be for each of these individuals:
(a) BBAa black with a band of yellow
(b) Ccbb brown
(c) ccBb no colour (albino)
example fur colour is controlled by several different genes.
C = the fur can have colour c = there is no colour (albino)
A = agouti = band of yellow on an otherwise dark hair a = no band of yellow
B = black fur b = brown fur
What would the phenotype be for each of these individuals:
(a) BBAa black with a band of yellow
(b) Ccbb brown
(c) ccBb no colour (albino)
