The Roles of Genes in Determining the Phenotype (Cambridge (CIE) A Level Biology): Flashcards
Exam code: 9700
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Define gene.
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Define gene.
A length of DNA that codes for a polypeptide (protein).
Define locus.
The specific position of a gene on a chromosome.
Define allele.
An alternative version of a gene.
Different alleles of the same gene are found at the same locus on homologous chromosomes.
What is the difference between a dominant and a recessive allele?
A dominant allele is expressed in the phenotype even when only one copy is present (heterozygous).
A recessive allele is only expressed when two copies are present (homozygous).
What does codominant mean?
Both alleles are fully expressed in the phenotype of a heterozygote — neither is dominant over the other.
Define linkage.
When two or more genes are located on the same chromosome, so they tend to be inherited together.
Define test cross.
A cross between an organism showing the dominant phenotype (unknown genotype) and a homozygous recessive individual, used to determine the unknown genotype.
What are the F1 and F2 generations?
F1: the first generation of offspring from the parental cross.
F2: the offspring produced by crossing F1 individuals together.
What is the difference between genotype and phenotype?
Genotype: the alleles an organism carries.
Phenotype: the observable characteristics, resulting from the genotype and the environment.
What is the difference between homozygous and heterozygous?
Homozygous: carrying two identical alleles of a gene (e.g. AA or aa).
Heterozygous: carrying two different alleles of a gene (e.g. Aa).
The specific position of a gene on a chromosome is called its .
The specific position of a gene on a chromosome is called its locus.
Define monohybrid cross.
A genetic cross that follows the inheritance of a single gene.
What phenotypic ratio is expected in the offspring of two heterozygotes (Aa × Aa)?
3 : 1 (dominant : recessive phenotype).
What is a Punnett square used for?
To predict the genotypes and phenotypes of offspring, by combining the possible gametes from each parent.
In codominance, how is the heterozygote expressed? Give an example.
Both alleles are expressed in the phenotype.
Example: the ABO blood group — genotype IAIB produces blood group AB, expressing both A and B antigens.
What is meant by multiple alleles? Give an example.
A gene that has more than two possible alleles in the population.
Example: the ABO blood group gene has three alleles — IA, IB and IO.
What is sex linkage?
When a gene is carried on a sex chromosome (usually the X chromosome).
Examples include haemophilia and red–green colour blindness.
Why are males more likely to show X-linked recessive conditions?
Males are XY, so they have only one X chromosome.
A single recessive allele on that X is therefore expressed, as there is no second X to carry a dominant allele.
A cross that follows the inheritance of a single gene is called a cross.
A cross that follows the inheritance of a single gene is called a monohybrid cross.
Define dihybrid cross.
A genetic cross that follows the inheritance of two different genes at the same time.
What phenotypic ratio is expected from a dihybrid cross between two heterozygotes (AaBb × AaBb), if the genes are unlinked?
9 : 3 : 3 : 1
What conditions are needed to get the standard 9 : 3 : 3 : 1 ratio?
Two genes on different chromosomes (unlinked)
Both parents heterozygous for both genes
Simple dominant/recessive inheritance at each gene
Define autosomal linkage.
When two genes are located on the same autosome (a non-sex chromosome), so their alleles tend to be inherited together rather than assorting independently.
How does autosomal linkage affect the offspring ratio of a dihybrid cross?
Linked alleles are usually inherited together, so there are more parental-type offspring and fewer recombinants than the expected 9 : 3 : 3 : 1 ratio.
How can recombinant offspring arise when genes are linked?
By crossing over during meiosis, which separates linked alleles and produces new combinations.
Define epistasis.
An interaction in which one gene affects or masks the expression of a different gene.
A genetic cross that follows the inheritance of two genes is called a cross.
A genetic cross that follows the inheritance of two genes is called a dihybrid cross.
Define test cross.
A cross between an organism showing the dominant phenotype (unknown genotype) and a homozygous recessive individual.
What is the purpose of a test cross?
To find out whether an organism showing the dominant phenotype is homozygous dominant or heterozygous.
Why is a homozygous recessive individual used in a test cross?
It can only pass on recessive alleles, so the offspring phenotypes depend entirely on the alleles from the unknown parent — revealing its genotype.
In a test cross, what does it mean if all offspring show the dominant phenotype?
The unknown parent is homozygous dominant (e.g. AA).
In a test cross, what does a 1 : 1 ratio of dominant to recessive offspring show?
The unknown parent is heterozygous (e.g. Aa).
A test cross is carried out using a recessive individual.
A test cross is carried out using a homozygous recessive individual.
What is the chi-squared (χ^2^) test used for in genetics?
To test whether the difference between observed and expected results is significant, or is simply due to chance.
What is the null hypothesis in a chi-squared test?
That there is no significant difference between the observed and expected results (any difference is due to chance).
What is the formula for the chi-squared test?
χ2 = Σ (O − E)2 / E
where O = observed value and E = expected value.
How do you calculate the degrees of freedom for a chi-squared test?
Degrees of freedom = number of categories − 1
How do you interpret χ2 against the critical value (p = 0.05)?
If χ^2^ ≥ critical value: the difference is significant — reject the null hypothesis.
If χ^2^ < critical value: the difference is not significant — it is due to chance, so accept the null hypothesis.
What probability level is normally used in biology for the chi-squared test?
p = 0.05 (the 5% significance level).
In a chi-squared test, the degrees of freedom equal the number of categories minus .
In a chi-squared test, the degrees of freedom equal the number of categories minus one.
In general, how do genes determine the phenotype?
Genes code for proteins (such as enzymes and structural proteins).
These proteins produce the organism's characteristics, so a mutation that alters a protein can alter the phenotype.
Explain the link between the TYR gene, tyrosinase and albinism.
The TYR gene codes for the enzyme tyrosinase, needed to make the pigment melanin.
A mutation produces a non-functional tyrosinase, so no melanin is made — causing albinism.
Explain the link between the HBB gene, haemoglobin and sickle cell anaemia.
The HBB gene codes for the β-globin chain of haemoglobin.
A base substitution changes one amino acid (glutamate → valine), producing haemoglobin S, which makes red blood cells sickle when oxygen is low.
Explain the link between the F8 gene, factor VIII and haemophilia.
The F8 gene codes for clotting factor VIII.
A mutation means little or no functional factor VIII is made, so the blood cannot clot properly — causing haemophilia.
Explain the link between the HTT gene, huntingtin and Huntington's disease.
The HTT gene codes for the protein huntingtin.
A mutation (an expanded repeated base sequence) produces an altered huntingtin protein that damages nerve cells, causing the progressive neurodegeneration of Huntington's disease.
The TYR gene codes for the enzyme , which is needed to make melanin.
The TYR gene codes for the enzyme tyrosinase, which is needed to make melanin.
What is the role of gibberellin in stem elongation?
Gibberellin promotes stem elongation, producing tall plants.
What does the dominant allele Le do in the gibberellin pathway?
It codes for a functional enzyme in the gibberellin synthesis pathway.
Active gibberellin is made, so the plant grows tall.
What does the recessive allele le do in the gibberellin pathway?
It codes for a non-functional enzyme, so little or no active gibberellin is made.
The plant is therefore short (dwarf).
What is the genotype of a dwarf pea plant in the gibberellin pathway?
Homozygous recessive (lele) — both alleles code for a non-functional enzyme, so no active gibberellin is produced.
The dominant allele codes for a functional enzyme in the gibberellin synthesis pathway.
The dominant Le allele codes for a functional enzyme in the gibberellin synthesis pathway.
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