Session 7: Genotype, phenotype & inheritance Flashcards
Which type of inheritance does Cystic Fibrosis show?
Autosomal dominant
What does homozygous mean?
2 alleles of a gene are the same
What does heterozygous mean?
2 alleles of a gene are different
What is a gene?
A stretch of DNA - the basic unit of hereditary passed from parent to child
How many alleles does each gene have?
2
What is an allele?
an alternative version of the same gene as the same position on the chromosome
What is phenotype?
the observable characteristics of an individual resulting from interaction of its genotype with the environment
What is genotype?
the combination of alleles for a specific gene
What does hemizygous mean?
there is only one allele (on X chromosome)
What does recessive mean?
non-dominant allele in a heterozygote (it is not expressed, in Aa, small a is recessive but not expressed in presence of dominant allele)
What does dominant mean?
dominant allele in a heterozygote determines the phenotype (ex Aa or AA)
What is an autosomal disease?
disease gene located on non-sex chromosome
What is a sex-linked disease?
gene on X or Y chromosome
What is codominance?
2 alleles are both expressed at the same time
Describe features of autosomal dominant diseases. Give an example.
- Single copy of gene leads to phenotype being expressed
- One parent must have the trait
- Heterozygotes affected (eg Hh), rarely found in homozygous form (HH)
- 2 heterozygotes have 50% chance of having affected offspring
- Doesn’t skip generations
- Example: Huntington’s disease
Describe features of autosomal dominant diseases. Give an example.
- Single copy of gene leads to phenotype being expressed
- One parent must have the trait
- Heterozygotes affected (eg Hh), rarely found in homozygous form (HH)
- 2 heterozygotes have 50% chance of having affected offspring
- Doesn’t skip generations
- Example: Huntington’s disease
Describe features of autosomal recessive diseases. Give an example.
- Able to skip generations
- Must have 2 affected chromosomes for disease to present (eg aa)
- Heterozygotes = carriers but unaffected (dominant allele dominates recessive)
- 2 heterozygotes = 25% chance of having affected offspring
- 2 homozygous = only have affected offspring
Example: Cystic fibrosis
What type of inheritance does sickle cell anaemia show?
Autosomal recessive disease
Describe features of X-linked recessive diseases.
- Affected gene on X chromosome
- For female to be affected, they must inherit recessive allele from mother AND father (2 copies of X)
- For male to be affected they only need one copy of recessive allele from mother
Why do affected males have heterozygous daughters in X-linked recessive inheritance?
They can only pass on 1 X copy of X chromosome and they do not pass on Y chromosome
Describe features of X-linked dominant diseases.
- Hemizygous males (Xx,Y) & heterozygous females (XX, Xx) are affected
- Heterozygous females (XX, Xx) = 50% chance of affected offspring
- Affected males cannot give trait to sons = because they only pass on Y chromosome to sons
- Affected males give trait to daughters = pass on single X chromosome that has dominant allele
Why do affected males not pass on the dominant x-linked trait to their sons?
Because they only pass on Y chromosome to sons
Why do affected males only pass trait to daughters in x-linked dominant diseases?
Because they only pass on Y chromosome to sons & they pass on single X chromosome that has dominant allele
Describe features of Y-linked dominant diseases.
- Disease causing allele on Y chromosome = so only expressed in males
- Carriers in all generations
What are the letters used to describe affected males in X-linked dominant disease? If D = have disease & d = not disease.
Affected males: XDY
Unaffected males: XdY
What are the letters used to describe affected females in X-linked dominant disease? If M = have disease & m = not disease.
Affected females: XMXm or XMXM
Unaffected females: XmXm
What are the letters used to describe affected females in X-linked recessive disease? If M = not disease & m = have disease.
Affected: XmXm
Unaffected: XMXm or XMM
What is mitochondrial inheritance?
Mitochondria contain a small circular chromosome, DNA (mtDNA) that is only inherited from mother
Why is mitochondrial DNA only inherited from mother?
Because mitochondria of egg used to produce all mitochondrial for cells of body
What happens if there is a mutation of mitochondrial DNA (mtDNA)?
cause malformation of proteins needed to regenerate large amount of ATP from oxidative phosphorylation needed by cells
What does mtDNA encode for?
Encodes proteins for oxidative phosphorylation
What is polygenic inheritance?
phenotype influenced by the activity of multiple genes rather than the activity of one single gene (monogenic)
Give an example of polygenic inheritance. How is it polygenic?
- Albinism
- More than 1 gene involved in the expression of albinism (2 copies of A gene)
- 2 albino people can have offspring that does not have albinism because there are 2 different copies of the A gene and so 2 more alleles (4 in total - A1, a1 & A2, a2)
- Homozygous recessive alleles must be present for albinism to be observed: a1a1 or a2a2.
What is gene linkage?
genes that are close together on the same chromosome are inherited together as they do not show independent assortment in meiosis
What happens for genes that are not linked?
Genes on diff chromosomes are not linked => they assort independently during meiosis and daughter cells will inherit them in set ratio 1:1:1:1
What are blocks of linkage formed on chromosomes called?
haplotypes
What is linkage disequilibrium?
loci on diff chromosomes are inherited together
What is the chromosomal locus?
The exact position of a gene on a chromosome
