Genetics & Inheritance 7.1 Flashcards
State three causes of genetic variation (3)
- Mutation
- Crossing over
- Independent segregation (of homologous chromosomes)
- Random fusion of gametes / fertilisation
What is meant by a genome? (1)
All the DNA in an organism
/ The total number of DNA bases / genetic material
In genetic crosses, the observed phenotypic ratios obtained in the offspring are often not the same as the expected ratios. Suggest two reasons why (2)
- small sample size;
- fusion/fertilisation of gametes is random;
- Crossing over / linked genes / sex-linkage;
- Epistasis;
- Lethal genotypes
What is a gene pool? (1)
All the alleles in a population
What does Hardy Weinberg’s equation predict?
- The frequency of alleles (on one gene);
- will stay constant over (many) generations;
- as long as there is:
no mutation, no selection, no genetic migration, random mating & large population
How do multiple alleles of a gene arise? (1)
- mutations;
- which are different / at different positon/loci of a gene
What is meant by a recessive allele? (1)
- Only expressed in the homozygote / not expressed in the heterozygote / not expressed if dominant present
In fruit flies, the genes for body colour and wing length are linked. Explain what this means (1)
- Both genes appear on the same pair of homologous chromosomes / autosomes
Define gene linkage (1)
Genes on the same chromosome
e.g A & B and a & b
Define epistasis (1)
The allele of one gene affects the expression of another in the phenotype
Describe why observed phenotypes don’t match expected values
- Fertilisation/fusion of gametes is random;
- small population/sample size;
- Selection advantage/disadvantage/Lethal alleles
Define codominance (1)
Both alleles expressed (equally) in the phenotype
Rules for dominant alleles (3):
- Affected offspring must have at least one affected parent
- Unaffected parents ONLY have unaffected offspring
- Both parents must be heterozygous if are affected and have unaffected offspring
Rules for recessive alleles (1):
- Unaffected parents can have affected offspring if they are heterozygous
Male offspring are more likely than females to show recessive sex-linked characteristics. Explain why (2)
- Recessive allele is always expressed in males / males have 1 recessive allele;
- Females need two recessive alleles / need to be homozygous recessive
What are the expected offspring phenotype ratios from heterozygous parents in:
1. Monohybrid
2. Dihybrid
3. Epistasis
4. Autosomal Linkage
Dominant : Recessive
1. 3:1
2. 9:3:3:1
3. 9:4:3 or 15:1 or 9:7
4. 3:1 (recombinant if no crossing over)
What is meant by the term phenotype? (1)
- Expression of genetic constitution/genome
due to interaction with environment
Explain how a single base substitution causes a change in the structure of a polypeptide (3)
- change in sequence of amino acids/primary structure;
- change in hydrogen/ionic/disulphide bonds;
- alters tertiary structure
What is an allele?
different forms of a gene at the same locus of a chromosome
(one gene can have many alleles)
How many alleles per gene do diploid organisms have?
2
What is meant by homozygous and heterozygous?
Homozygous = both alleles are either dominant or recessive
Heterozygous = one allele is dominant and one is recessive
Define monohybrid inheritance (1)
One phenotypic characteristic is controlled by a single gene
Define dihybrid inheritance (1)
Two different characteristics are controlled by two different genes on DIFFERENT CHROMOSOMES at the SAME TIME.
What is meant by sex-linkage? (1)
Where an allele is located on one sex chromosome, so its expression depends on the sex of the individual
Which parent do males inherit sex-linked characteristics from?!
Mother - Y chromosome can only be inherited from father, so the mother must be a heterozygous carrier for sex-linked alleles
Dads + Daughters =
Mums + Sons =
- Recessive
- Dominant
What gametes would be produced from sex-linked genotypes: (Mother) XH Xh and (Father) XH Y
XH XH - Normal female
XH Xh - Carrier female
XH Y - Normal male
Xh Y - Affected male
Describe autosomal linkage (2)
Two or more genes are located on the same (NON-SEX) chromosome.
Only one homologous pair is needed for all 4 alleles to be present
(Non-linked genes need two homologous pairs)
Describe epistasis (1)
One (non-linked) gene masks the expression of another (non-linked) gene
E.g Allele for baldness affects expression of allele for hair colour?
Define the two types of epistasis
Recessive epistasis: two homozygous recessive alleles (aa) mask expression of another allele
Dominant epistasis: one dominant allele (B) affects expression of multiple alleles
e.g B for baldness affects hair colour, length, curls, etc.
Define the Chi-squared test (1)
Statistical test that finds out whether a difference between observed and expected data is due to random chance or not.
DATA IS CATEGORIC!
Chi-Squared test MUST HAVE (4):
- Data in categories
- Large sample size
- Only raw data (no %)
- No data values = 0
How is a Chi-squared test used in inheritance? (1)
Compare expected and observed phenotypic ratios (shows what alleles are inherited)
Formula for Chi-Squared test
χ2 = (O - E)2
∑ ———
E
O = Observed
E = Expected
∑ = Sum
How is the Chi-Squared test carried out? (3)
- Formula from table of categoric data (Χ2 = ∑ (O – E)2 divided by E)
- Find appropriate degrees of freedom as N - 1 (N = number of outcomes, e.g head or tails)
- Compare formula number with critical value (P)
- If number is greater than/equal to critical value, there is a significant difference between O and E / NOT due to chance
Use your knowledge of gene linkage to explain the offspring of GgNn and ggnn (4)
Work out gametes
1. GN and gn are linked;
2. GgNn parent produces mainly GN and gn gametes;
3. Crossing over produces few Gn and gN gametes (as rare);
4. (so) fewer Ggnn and ggNn individuals.
What is the expected ratio of offspring phenotypes from a heterozygous parent (GgNn) crossed with a homozygous parent? (ggnn)
1:1:1:1
Heterozygous produces 4 different types of gametes, homozygous only produces 1 type, so 4 types of offspring produced.
Which statistical test could the scientist use to determine whether his observed results were significantly different from the expected results? (2)
- Chi-squared (test);
- (Because) the data is categoric.
Define a linkage group (1)
A linkage group is:
A set of genes on the same chromosome which tend to be inherited together
