Inheritance (A Level Only)
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Notas de aula
Key Terms in Genetics
- A gene is a DNA sequence that codes for a polypeptide or protein, located at a specific locus on a chromosome.
- Different versions of a gene are called alleles; they have slightly different base sequences but occupy the same locus.
- The genotype is the alleles an organism possesses; the phenotype is the observable characteristics resulting from the genotype and the environment.
- Individuals can be homozygous (two identical alleles) or heterozygous (two different alleles) for a gene.
- A dominant allele is always expressed in the phenotype, even in heterozygotes; a recessive allele is only expressed when no dominant allele is present (i.e. in homozygotes).
- Codominance occurs when both alleles in a heterozygote are fully expressed, so features of both alleles are seen in the phenotype.
- In codominance, genotype notation uses a capital letter for the gene and superscript letters for alleles, e.g. Iᴬ, Iᴮ, Iᴼ for human blood groups.
Monohybrid Crosses
- Monohybrid inheritance tracks the inheritance of one gene with two alleles.
- During gamete formation, each allele from a homologous pair has an equal chance of being passed on, so the zygote has an equal probability of inheriting either parental allele.
- Genetic diagrams (especially Punnett squares) are used to predict offspring genotypes and phenotypes.
- A cross between two heterozygotes (e.g. Bb × Bb) gives a predicted phenotypic ratio of 3 dominant : 1 recessive and a genotypic ratio of 1 BB : 2 Bb : 1 bb.
- Predictions are based on probability; observed ratios may differ due to random fertilisation of gametes.
- For codominant alleles, there are more possible phenotypes, so predicted ratios differ from simple dominant/recessive crosses.
- In sex-linked inheritance, males (XY) have one copy of X-linked genes, so are more likely to express recessive traits; females (XX) can be unaffected, carriers, or affected.
Sex Linkage and Haemophilia
- Sex-linked genes are located on the X chromosome (rarely on the Y chromosome).
- Males have one X and one Y chromosome, so they have only one allele for X-linked genes; they cannot be carriers of recessive X-linked traits.
- Females have two X chromosomes, so they can be homozygous dominant (unaffected), heterozygous (carriers), or homozygous recessive (affected).
- Haemophilia is a recessive X-linked disorder caused by a gene coding for Factor VIII, a protein essential for blood clotting.
- Males need only one copy of the recessive allele (XᵃY) to be affected; females must inherit two copies (XᵃXᵃ) to be affected.
- A carrier female (XᴬXᵃ) crossed with a normal male (XᴬY) gives a 1:1:1:1 ratio of normal female : carrier female : normal male : affected male.
- Males cannot pass X-linked traits to their sons because they only pass on the Y chromosome to sons.
Dihybrid Crosses and Linkage
- Dihybrid crosses track the inheritance of two genes simultaneously, involving more genotype and phenotype combinations.
- In genotype notation, write both alleles of one gene first, then both alleles of the second gene (e.g. YyGg); do not mix alleles between genes.
- Dihybrid crosses assume independent assortment of genes during meiosis, giving a typical 9:3:3:1 phenotypic ratio when two heterozygotes are crossed.
- Autosomal linkage occurs when two or more genes are on the same chromosome; they do not assort independently and tend to be inherited together, maintaining the parental allele combination.
- Linked alleles are often written in brackets to show they are inherited together, e.g. (FG)(FG) instead of FFGG.
- Linkage reduces variation and alters expected offspring ratios, so they deviate from standard dihybrid predictions.
- Crossing over during meiosis can produce recombinant offspring, so not all offspring show the parental combinations.
Epistasis
- Epistasis occurs when one gene affects the expression of another gene, involving two genes on different chromosomes influencing the same trait.
- The interaction between alleles at different loci determines the resulting phenotype.
- Epistasis must be considered when predicting phenotypic ratios in genetic crosses.
- Example: in mice, the allele for black fur (B) is dominant to brown (b), but a homozygous recessive genotype (cc) at a second gene prevents pigment production, resulting in albino mice.
- Example: in pigeons, the dominant allele F stops grey feather production even if the allele R for grey pigment is present, so only rrff birds have grey feathers.
- Epistasis can produce modified phenotypic ratios that do not fit standard Mendelian predictions.
Test Crosses
- A test cross determines the genotype of an individual showing a dominant phenotype by crossing it with a homozygous recessive individual.
- The homozygous recessive individual has a known genotype (e.g. hh, rr, bb) and expresses the recessive phenotype.
- For a monohybrid test cross: if no recessive offspring appear, the unknown is homozygous dominant; if at least one recessive offspring appears, the unknown is heterozygous.
- For a dihybrid test cross: no recessive phenotypes for either gene means the unknown is homozygous dominant for both genes.
- If at least one recessive phenotype appears for one gene only, the unknown is heterozygous for one gene and homozygous dominant for the other.
- If at least one recessive phenotype appears for both genes, the unknown is heterozygous for both genes.
Investigating Genetic Ratios
- Genetic diagrams predict offspring genotypes and phenotypes using known parental genotypes and meiosis, but actual ratios may differ due to random fertilisation.
- Model organisms such as Fast Plant® (rapid-cycling Brassica rapa) and Drosophila (fruit flies) are used because they have short life cycles and easily identifiable traits controlled by single genes.
- In Fast Plant®, stem colour is controlled by a single gene: A = dominant (anthocyanin present, purple stem), a = recessive (no anthocyanin, green stem).
- A cross between two heterozygous Fast Plants (Aa × Aa) gives a predicted 3 purple : 1 green ratio.
- In Drosophila, wing length is controlled by a single gene: L = dominant (long/wild-type wing), l = recessive (vestigial wing).
- A cross between two heterozygous Drosophila (Ll × Ll) gives a predicted 3 wild-type : 1 vestigial ratio (≈75% : 25%).
- In a dihybrid cross, a 9:3:3:1 ratio means four possible phenotypes with probabilities of 56.25%, 18.75%, 18.75% and 6.25%.
The Chi-squared Test
- The chi-squared test determines whether there is a significant difference between observed and expected results in an experiment.
- It is used when data is categorical, i.e. falls into distinct groups.
- The chi-squared value is calculated using the formula χ² = Σ(O − E)² ÷ E, where O = observed and E = expected.
- The degrees of freedom are calculated as the number of classes minus 1.
- Biologists generally use a probability level of 0.05 (5%); the critical value is read from a table using this probability and the degrees of freedom.
- If χ² ≥ critical value, there is a significant difference not due to chance; reject the null hypothesis.
- If χ² < critical value, there is no significant difference; accept the null hypothesis as any difference is due to chance.
- A significant difference may suggest an unaccounted factor such as linkage between genes.
Slides
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Questões de prática
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1.What is the definition of a gene?
Easy- AA DNA sequence that codes for a polypeptide or protein
- BA length of DNA that codes for a carbohydrate
- CA protein that controls a characteristic
- DA section of RNA that codes for a protein
2.Which of the following best defines the term 'genotype'?
Easy- AThe observable characteristics of an organism
- BThe alleles possessed by an organism
- CThe physical location of a gene on a chromosome
- DThe number of chromosomes in a cell
3.In genetics, what is a sex-linked characteristic?
Easy- AA characteristic controlled by a gene located on the X chromosome
- BA characteristic controlled by a gene on an autosome
- CA characteristic that is only expressed in males
- DA characteristic controlled by multiple genes
4.Which of the following correctly describes codominance?
Medium- ABoth alleles in a heterozygous genotype are fully expressed, so features of both are observed
- BOne allele is always expressed in the phenotype, masking the other
- CThe phenotype is intermediate between the two homozygous phenotypes
- DOnly one allele is present in the genotype
5.Which of the following is the correct definition of epistasis?
Medium- AOne gene affects the expression of another gene
- BTwo genes on the same chromosome are inherited together
- CBoth alleles in a heterozygote are fully expressed
- DA gene located on the X chromosome
6.In a monohybrid cross between two heterozygous individuals (Aa x Aa), what is the expected phenotypic ratio of dominant to recessive offspring?
Medium- A3:1
- B1:1
- C1:2:1
- D9:3:3:1
7.A test cross is performed to determine the genotype of an individual showing a dominant phenotype. If all offspring show the dominant phenotype, what is the genotype of the unknown individual?
Medium- AHomozygous dominant
- BHeterozygous
- CHomozygous recessive
- DCannot be determined
8.In a dihybrid cross where both parents are heterozygous for two independently assorting genes, what is the expected phenotypic ratio?
Medium- A9:3:3:1
- B3:1
- C1:1:1:1
- D1:2:1
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