Modifications to Mendel Flashcards
(26 cards)
Define incomplete dominance
The heterozygous genotype causes a phenotype that is an intermediate between the two homozygous genotypes. E.g. AA - red, aa - white, Aa - pink flowers. Palomino horse
Give an example of incomplete dominance in Humans
Individuals with Tay Sachs have no hexosaminidase enzyme activity. Homozygous dominant individuals have high activity, heterozygotes have intermediate activity - pre-natal diagnosis
What is Tay-Sachs disease?
Autosomal recessive disorder common in Ashkenazi Jews (1/25 carriers) that causes loss of motor skills, blindness, paralysis, and seizures. Appears at about 6 months and children typically do not survive past early childhood. Mutations in the HEXA gene lead to deficient hexosaminidase A enzyme and neurodegeneration.
Define codominance
Loci at which the phenotypic effects of a gene’s allele are fully and simultaneously expressed in the heterozygote. E.g. A red cow with white spots.
Give an example of codominance in humans
Sickle cell trait: heterozygotes for the haemoglobin beta gene produce both normal and abnormal, sickle shaped haemoglobin, conferring some resistance to the parasitisation of erythrocytes. Can be diagnosed with electrophoresis.
Give examples of multiple alleles at one locus
Drosophila eye colour, clover leaf patterns determined by five alleles. Human eye colour (at several loci) and ABO blood groups.
How do lethal alleles affect phenotypic frequencies?
For example yellow mice - cross a yellow mouse and wildtype and the ratio is 1:1 as expected for autosomal dominance - yellow mouse was a heterozygote. However, if yellows are crossed, ratio is 1:2 wildtype:yellow… homozygote is missing as embryos die before developing.
Give three examples of a lethal homozygous allele
Manx cat, no tail. If a cat inherits two copies of the Manx allele, it is lethal. The same is true for brachydactyly. Achondroplasia is similarly a dominant condition which is lethal in homozygotes.
Define Pleiotropy
Where one gene affects many phenotypes.
Give a non-human example of pleiotropy
All blue-eyed white cats are deaf, due to a fault in the melanin pathway. Tabby cats
are particularly aggressive, pleiotropy on coat colour and behaviour.
Give an example of pleiotropy in humans
Sickle cell anaemia: one base change - one amino acid glutamate to valine - haemoglobin shape changes and erythrocytes sickle. But this has many effects: blocking capillaries -> heart failure, paralysis, bossed skull, brain damage, enlarged spleen, malaria resistance in heterozygotes
Describe sex limitation of alleles
when the expression of a phenotype is dependent on the individual’s sex, due to sex differences in hormones which interact with gene loci. In peacocks, both males and
females have the genes which code for an elaborate tail, but they are only expressed in males because only they produce testosterone.
Give examples of sex limitation in humans
Secondary sexual traits in males such as beards and sex-limited male pattern baldness only effect males because the traits are only expressed in the presence of testosterone. Many mental disorders are more common and more sever in males, such as schizophrenia and ADHD. BRCA1 (BRCA=BReast CAncer) mutations are associated with an increase in breast
cancer in women because of gene interaction with cell division genes and oestrogen.
Define gene interaction
Several genes affect one character.
Genetics is often seen as one character, one gene. In fact, there are many cases in which alleles at one locus alter the phenotype produced by an allele at another locus.
Describe the gene interactions in mouse coat colour
Many inbred lines. At least 5 loci in control. A locus: Agouti is the dominant wildtype - hairs with yellow bands. Recessive homozygosity removes the band.
B locus: BB/Bb - black, bb - brown, A-bb - cinnamon - gene interaction.
C locus: Epistatic - controls pigment presence. CC/Cc - Pigmented, cc - prevents A & B locus expression causing albinism.
S locus: control distribution of colour in the coat. SS/Ss - no spots, ss - piebald - large patches of colour.
Give an example of gene interaction in humans
KIT gene for cell division and haematopoiesis (involved in cancer). Mutations in KIT can lead to piebaldism in humans - pleiotropy.
Describe gene interaction between Secretor and ABO loci in humans
Se locus (FUT2 gene): Encodes an enzyme that allows expression of ABO antigens in bodily fluids (saliva, semen, etc.). SeSe/Sese + A/B/AB do this, while sese homozygotes can’t as the sese genotype masks the expression of the ABO locus in secretions.
Define Complementation
when you cross two strains of an organism with the same phenotype but
different homozygous alleles to produce a new phenotype. Foxglove normally purple petals, but some populations have white individuals - homosygous recessive. Cross white plants from different populations - F1 plants are purple, not white; and a ratio of 9 purple to 7 white in the F2. Two loci involved; and homozygosity for either = white. A chain of reactions produces purple, and a break at any point = white
Describe complementation tests
Complementation tests are used to show whether two alleles at a locus are involved, or two (or more) different loci E.g. drosophila eye colour. A line inbred for homozygous green eyes and one for blue eyes are crossed. If F1 have either eye colour, then these are two alleles at one locus. If F1 have red eyes, they’re heterozygous at two different eye colour loci and the recessive alleles aren’t expressed as the loci complement.
What is a complementation group?
A group of mutant alleles that all fail to complement each other — meaning they all affect the same gene, if present together, the phenotype is mutant. Recessive alleles that complement affect different loci - they’re in different complementation groups - and can be present together in a wildtype individual.
What’s the importance of complementation groups
help define how many distinct genes are involved in a particular biological pathway or phenotype. E.g. f you mutagenize flies and isolate 6 mutants with a wing defect, and complementation tests show they fall into 3 complementation groups, that means 3 separate genes are required for normal wing development.
Complementation and human hearing
Hearing phenotype: Sound → A → B → C → D → E → F → Perception of sound.
Genotypes:
Parents
AA → BB → cc → DD → EE → FF – deaf
AA → BB → CC → DD → ee → FF – deaf
Offspring
AA → BB → Cc → DD → Ee → FF – can hear
How do genes and environment interact in human hearing?
Being exposed to loud noises over long periods of time causes deafness, but individuals with darker skin are less likely to go deaf due to exposure to loud noises than individuals with whiter skin.
What is the difference between non-syndromic and syndromic deafness?
Non-syndromic deafness affects only hearing, with no other symptoms. Syndromic deafness occurs as part of a broader syndrome. Can affect heart, brain, kidneys, eyes, joints, genitals.
