Genes and inheritance Flashcards
What is a gene?
A length of DNA at a particular location on a chromosome that codes for a particular polypeptide/protein and determines a particular feature.
What is an allele?
A particular version of a gene.
What is a genotype?
The combination of of alleles present in genome of organism.
What is a phenotype?
Characteristic of an organism which result both from the genes it possesses and the environment in which it has developed. An observable or measurable characteristic.
What is a dominant allele?
An allele that, if present, will always be expressed in a phenotype, even if only one copy is present.
What is a recessive allele?
A characteristic in which the allele responsible is only expressed in the phenotype if 2 copies are present (i.e. no dominant alleles).
What is codominance?
A characteristic where both alleles are expressed in a phenotype of a heterozygote.
What is a locus?
The specific location of a gene on a chromosome.
What is a homozygote?
An organism that carries 2 copies of the same allele in genome.
What is a heterozygote?
An organism that carries 2 different alleles.
What is a carrier?
A person carrying an allele that is not expressed (recessive allele in heterozygotes) but is capable of passing on allele to offspring.
What is linkage?
When 2 or more genes are located on the same chromosome and are always inherited together, unless crossing over occurs in meiosis.
What is sex-linkage?
A characteristic which is controlled by a gene which is located on a sex chromosome (either X or Y).
How is a genetic diagram constructed?
A genetic diagram usually has the following categories:
- P1 Phenotype: Phenotypes of parents.
- P1 Genotype: Genotype of parents.
- P1 Gametes: All possible combinations of alleles in gametes produced by parents (with circle around each)/
- Fertilisation: Can use punnet square or genetic cross diagram to determine all possible genotypes of offspring.
- F1 Genotype: All possible genotypes of offspring.
- F1 Phenotype: All possible phenotypes of offspring.
- Phenotype/genotype ratio: Ratio of different phenotypes/genotypes theoretically produced by parents.
What are examples of codominance?
- Sickle-cell anaemia.
- Roan cattle.
- Human blood groups.
What causes sickle-cell anaemia?
- β-strands in haemoglobin abnormal due to slightly different primary structure.
- Deoxygenated haemoglobin becomes insoluble and crystallises.
- Red blood cells become sickle-shaped.
- Sickle-shaped red blood cells are unable to squeeze through capillaries so get stuck and block capillaries, reducing blood-flow to vital organs.
- Organs suffer from oxygen deprivation and are damaged.
What are the genetics behind sickle-cell anaemia?
- Sickle-cell anaemia is codominant.
- H^A (normal haemoglobin) and H^S (sickle-cell haemoglobin) both code for expressed genes.
- An individual with an H^A H^A genotype are people with normal haemoglobin.
- An individual with an H^A H^S genotype are carriers. Some of their haemoglobin is abnormal while the rest is normal, causing no symptoms.
- An individual with an H^S H^S genotype are sufferers of the disease. All their haemoglobin are abnormal, so all red-blood cells are sickle-shaped, causing sickle-cell anaemia.
What are the genetics behind roan cattle?
- C^W codes for white hair.
- C^R codes for red hair.
- Individuals with C^W C^W genotype only produce proteins for white hair, so are white.
- Individuals with C^W C^R genotype produce proteins for both red hair and white hair, so are roan.
- Individuals with C^R C^R genotype only produce proteins for red hair, so are red.
What evidence is there in a family pedigree diagram to suggest that there is a sex-linked disease in the family?
When a higher majority of one sex has possessed the disease than the other throughout the family history.
What is the nature of sex-linked inheritance?
- If condition is Y-linked, only males will inherit the disease.
- If the condition is X-linked, both females and males can inherit the disease, but females need to inherit 2 faulty X chromosomes in order to acquire disease whilst males only need 1.
- Males cannot be carriers of the disease as they only have 1 X chromosome, so they either have the disease or they don’t.
What evidence is there in a family pedigree to indicate the condition is recessive?
When 2 unaffected individuals produce an affected offspring. This indicates that both parents were carriers of the disease.
What is monohybrid inheritance?
Inheritance of a characteristic controlled by a single gene.
What is dihybrid inheritance?
Inheritance of 2 characteristics controlled by 2 different genes at 2 different loci.
What are the 2 types of dihybrid inheritance?
- Unlinked: Two genes are on 2 different chromosomes and are inherited separately, independent of one-another.
- Linked: Two genes are on the same chromosome and are always inherited together. The same alleles are also always inherited together unless crossing over occurs (no random assortment).
