3.3.6 Inheritance Flashcards
Define gene
A sequence of bases on a DNA molecule that codes for a protein (polypeptide), which results in a characteristic
Define Genotype
The genetic constitution of an organism
(i.e. the alleles an organism has)
Define Phenotype
The expression of the genetic constitution and its interaction with the environment
(i.e. an organism’s characteristics)
Define Allele
A different version of a gene
Explain what is meant by a recessive allele
Only expressed in phenotype when 2 alleles are present
Explain what is meant by a dominant allele
Is always expressed in the phenotype
Explain what is meant by co-dominant alleles
Both alleles are expressed in the phenotype
Define Homozygous
An organism that carries 2 copies of the same allele
Define Heterozygous
An organism that carriers 2 different alleles
Define Carrier
Person carrying an allele which isn’t expressed in phenotype but can be passed on to offspring
Define Locus
Fixed position of gene on a chromosome
(Alleles of gene are found at same locus on each chromosome in a pair)
Define Autosome
Any chromosome that is not a sex chromosome
Define Allosome
Sex chromosome
What is monohybrid inheritance?
Inheritance of a characteristic controlled by a single gene
Draw a monohybrid cross showing how wing length is inherited in fruit flies when the parent’s genotypes are NN x nn. Include the phenotypes and phenotypic ratio.
N - normal wings allele
n - vestigial (little) wings allele
Draw genetic diagram when the paren’ts genotypes are HNHS x HNHS (i.e. crossing 2 parents with sickle-cell trait (heterozygous))
HN = normal haemoglobin
HS = sickle haemoglobin
In the ABO blood group system in humans there are 3 alleles for blood type:
- IO is the allele for blood group O
- IA is the allele for blood group A
- IB is the allele for blood group B
Allele IO is recessive. Alleles IA and IB are codominant - people with genotype IAIB will have blood group AB.
Draw a genetic diagram that shows a cross between a heterozygous person with blood group A and a heterozygous person with blood group B.
What do dihybrid crosses show?
Show how 2 genes are inherited are once
Draw a dihybrid cross diagram for when the both parents’ gentotype is RrYy. Include the phenotype and the phenotypic ratio.
- R - round seed
- r - wrinkled seed
- Y - yellow seed
- y - green seed
When are characteristics said to be sex-linked?
When the allele that codes for a characteristic is located on a sex chromosome
Most genes on sex chromosomes are only carried on ___ chromosomes
X
(X-linked genes)
Explain why males are more likely than females to show recessive phenotypes for genes that are sex-linked
- Males only have 1 X = 1 allele for sex-linked genes
- ∵ have only 1 copy, express characteristic of allele even if its recessive
Colour blindness is a sex-linked disorder caused by a faulty allele carried on the X chromosome
Given that the female parent is a carrier and the male is unaffected, draw monohybrid diagram (n = faulty allele for a colour vision)
What are autosomal genes?
Genes located on the autosomes
Genes on the same autosome are said to be ____
linked
Why are autosomal genes said to be linked?
- ∵ they’re on the same autosome, they’ll stay together in independent segregation of chromosomes in meiosis I
- And their alleles will be passed on to offspring together
(Only doesn’t occur if crossing over splits them up 1st)
The closer together 2 genes are on autosome, the more …
closely they’re linked
Why is that the more closer together 2 genes are on autosome, the more closely they’re linked?
∵ reduces the chance of genes being separated during crossing over
What happens to the phenotypic ratio expected in offspring when 2 gene are autosomally linked?
- It changes - won’t get the phenotypic ratio you expect in offspring of cross
- e.g. Dihybrid cross between 2 heterozygous parents = 9:3:3:1 ratio in offspring
- Instead phenotypic ratio more likely to be like monohybrid cross between heterozygous parents (3:1)
Why is the phenotypic ratio different when 2 genes are autosomally linked?
∵ higher proportion of offspring will their parents’ (heterozygous) genotype and phenotype
Explain the difference in the phenotypic ratios
- NR and nr linked (in NnRr parent)
- NnRr parent produces mainly NR and nr gametes
- So fewer Nnrr and nnRr
- Crossing over produces some Nr and nR gametes
What is epistasis?
When an allele of 1 gene masks (blocks) the expression of the alleles of other genes
Name the 3 types of epistasis
- Recessive epistasis
- Dominant epistasis
- Complementary epistasis
Describe dominant epistasis
Having at least one copy of the dominant epistatic allele masks (blocks) the expression of the other gene
Dominant Epistasis
State the phenotypic ratio when you cross a homozygous recessive parent with a homozygous dominant parent
12 : 3 : 1
dominant epistatic : recessive epistatic dominant other : recessive both
Squash colour is controlled by 2 genes - the colour espistatic gene (W/w) and the yellow gene (Y/y). The no-colour, white allele (W) is dominant over the coloured allele (w). The yellow gene has the dominant yellow allele (Y) and the recessive green allele (y).
Draw a genetic digram crossing WwYy with WwYy. Include the phenotypes and phenotypic ratio.
Describe recessive epistasis
Having 2 copies of the recessive epistatic allele masks (blocks) the expression of the other gene
Recessive Epistasis
State the phenotypic ratio when you cross a homozygous recessive parent with a homozygous dominant parent
9 : 3 : 4
dominant both : dominant epistatic recessive other : recessive epistatic
Flower pigment in a plant is controlled by 2 genes. Gene 1 codes for a yellow pigment (Y = dominant yellow allele) and gene 2 codes for an enzyme that turns the yellow pigment orange (R = dominant yellow allele). Cross YyRr x YyRr. Include phenotypes and phenotypic ratios.
When do you use the chi-squared (χ2) test?
Used to see if results of experiment support a theory
State the chi-squared formula
- O = Observed result
- E = Expected result
To find if there’s a significant difference between your observed and expected results, you need to compare χ2 value to ____ ____
critical value
If χ2 ≥ critical value, there’s a
- Significant difference
- (Something other than chance causing difference)
- Null hypothesis can be rejected
If χ2 < critical value, there’s …
- NO significant difference
- Accept null hypothesis
Describe how you work out the degrees of freedom
Number of classes (no. of phenotypes) - 1
e.g. 2 - 1 = 1
How do multiple alleles of a gene arise? (2)
- Due to mutations
- Which occur in different positions in the gene
At extremities (e.g. legs/arms) temperature is _____
lower
Explain one piece of evidence from the diagram which proves that the allele for Tay-Sachs disease is recessive (2)
- 3 & 4 produce 9/11 who are affected
- Both 3 and 4 are heterozygous (carry recessive allele)
Explain one piece of evidence from the diagram which proves that the allele for Tay-Sachs disease is not on the X chromosome (2)
- 11 is affected, 3 is not
- (If on X) 11 / affected female would not receive the recessive allele on X chromosome / Xt from 3 / father
Explain why sex-linked conditions are more common in males than in females (3)
- Males have XY chromosomes and females have XX chromosomes
- Recessive allele on X chromosome has no (equivalent) allele on Y chromosome
- Male needs to inherit one recessive allele whereas female needs to inherit two
Suggest one reason why observed ratios are often not the same as expected ratios (1)
- Due to chance
- Fertilisation is random
How is variation genetically controlled? (1)
Polygenes / several genes
Extension: State the genotype of the heterozygous parent using the correct notation
