4.3 Theoretical Genetics Flashcards
Define genotype
The complete set of genetic information of an organism; the allele combination of an organism
Define phenotype
The set of characteristics of an organism resulting from its genotype and environmental factors
Define dominant allele
An allele that has the same effect on the phenotype whether it is present in the homozygous or heterozygous state
Define recessive allele
An allele that only has an effect on the phenotype when present in the homozygous state
Define codominant alleles
Pairs of alleles that both affect the phenotype when present in a heterozygote
Define locus
The particular position of a gene on homologous chromosomes (plural: loci)
Define homozygous
Having two identical alleles of a gene
Define hetrozygous
Having two different alleles of a gene
Define carrier
An individual that has one copy of a recessive disease causing allele in their genotype which is not expressed in their phenotype. Carriers can pass this allele onto offspring.
Define test cross
Testing a suspected heterozygote by crossing it with a known homozygous recessive
Do you know how to do Punnett grids?
You better
Describe how pairs of non-recessive alleles can affect the genotype
- Share codominance (be expressed equally in the phenotype)
- Share incomplete dominance (neither is fully expressed in the phenotype, resulting in blending)
- Demonstrate a dominance order (e.g. allele A > allele B > allele C)
Explain how sex chromosomes control gender by referring to the inheritance of X and Y chromosomes in humans
• Humans have 23 pairs of chromosomes.
• First 22 are autosomes - each chromosome pair has the same genes and structure.
• The 23rd pair are sex chromosomes which determine gender.
– XY=Male
– XX=Female
– Y chromosomes shorter than X chromosome
• Gender is determined by sperm, as female eggs will always contain X, but male sperm could contain X or Y
Define sex-linkage
Sex linkage refers to when a gene controlling a characteristic is found on only one of the sex chromosomes.
• Sex linked genes are usually X-linked, as very few genes exist on the shorter Y chromosome
• If the gene is h, represented as Xh because it’s on the X chromosome (XH for dominant)
Explain the presence of sex-linked genes
• The Y chromosome is much shorter than the Y chromosome, and contains less genes.
• X and Y chromosomes have a homologous section where you will find the same genes (alleles).
• A male will only have one allele for chromosomes which appear on the non-homologous section of the X chromosome.
Males more likely to display recessive x-linked genetic abnormalities.
• The genes for red-green colour blindness and haemophilia are x-linked recessive.
• If a female has this gene on only one of her chromosomes, the other allele will dominate it.
• If a male has this gene, he has no alternative allele.
Describe the inheritance of colour blindness and haemophilia as examples of sex linkage
- Colour blindness and haemophilia are both examples of X-linked recessive conditions.
- The gene loci for these conditions are found on the non-homologous region of the X chromosome (they are not present of the Y chromosome).
- As males only have one allele for this gene they cannot be a carrier for these conditions.
- This also means they have a higher frequency of displaying these conditions.
- Males will always inherit an X-linked recessive condition from their mother.
- Females will only inherit an X-linked recessive condition if they receive a recessive allele from both parents.
Explain how human females can be heterozygous or homozygous with regard to x-linked genes
As human females have two X chromosomes (and therefore two alleles for any given X-linked gene), they can be either homozygous or heterozygous with regard to sex-linked genes.
Explain how females can be carriers for X-linked recessive alleles
- An individual with a recessive allele for a disease condition that is masked by a normal dominant allele is said to be a carrier.
- Carriers are heterozygous and can potentially pass the trait on to the next generation, but do not suffer from the defective condition themselves.
- Females can be carriers for X-linked recessive conditions because they have two X chromosomes - males (XY) cannot be carriers.
- Because a male only inherits an X chromosome from his mother, his chances of inheriting the disease condition from a carrier mother is greater.
Outline the rules of a pedigree chart
A pedigree is a chart of the genetic history of a family over several generations
• Males are represented as squares, while females are represented as circles
• Shaded symbols means an individual is affected by a condition, while an unshaded symbol means they are unaffected
• A horizontal line between a man and woman represents mating and resulting children are shown as offshoots to this line
State the genotype of blood type A
I^A I^A or I^A i
State the genotype of blood type B
I^B I^B or I^B i
State the genotype of blood type AB
I^A I^B
State the genotype of blood type O
i i
