Topic 12 Flashcards
What is genotype and phenotype?
genotype is the genetic makeup
phenotype is the characteristic
Define gene and allele
gene- a stretch of DNA sequence at a specific chromosomal location that carries the code for a polypeptide or untranslated RNA
allele- a variant of a gene
What are the meanings of homozygous, heterozygous and hemizygous?
homozygous two alleles of a gene are the same heterozygous two alleles of a gene are different hemizygous only one allele of a gene on the X chromosome (males)
What is the dominant and co-dominant allele?
dominant- in a heterozygote determines the phenotype
co-dominant- neither allele in a heterozygote is dominant and the phenotype is new
Explain the example of human ABO blood types
the gene for human ABO blood has three alleles
allele A and allele B are dominant over allele O
allele A and allele B are co-dominant
4 phenotypes; blood group A, B, AB and O
6 genotypes; Ia Ia or Ia Io, Ib Ib or Ib Io, Ia Ib, Io Io
(I is the gene for blood)
Recall the key for a pedigree
square- man circle- woman shaded- affected half shaded- heterozygote rhombus- sex unknown strike through- dead *twins* two lines between people- consanguineous marriage (blood relation marriage)
siblings in age order from left to right
males left and females right in relationship
label every generation with Roman numerals
label every individual of a generation in numbers left to right
shading indicates phenotype
What is autosomal recessive?
heterozygotes unaffected
affected are homozygous recessive
two affected individuals will only have affected offspring
two carriers have 25% chance affected offspring
males and females equally affected
disease can skip generation, can come out of nowhere
both parents of affected individual are carriers
eg cystic fibrosis
What is autosomal dominant?
males and females are equally affected
will not skip generations
every affected individual has 50% of affected offspring
assume only heterozygotes affected AD diseases rarely found in homozygous state
eg Huntington’s disease
What is X linked recessive?
males and females unequally affected
every affect hemizygous male has at least a carrier mother
every affected female will have an affected father and a carrier mother
affected males will have at least heterozygous daughters
What is X linked dominant?
males and females equally affected
hemizygous males and heterozygous females affected
heterozygous females have 50% chance of affect offspring
affected males cannot give trait to sons
affected males will give trait to all daughters
What is Y linked?
only males affected
affected males will give trait to their sons
not very common
What is mitochondrial inheritance?
all individuals will inherit from their mother
this is because only the oocyte has mitochondria as the mitochondria from sperm can’t enter the cell as if they do they are degraded
What is polygenic inheritance?
where more than one gene can be involved in producing a phenotype
genes on the same chromosome are said to be linked
genes on different chromosomes are said to be not linked
linked genes do not show independent assortment at meiosis
recombination frequency between two linked genes is dependent on the distance between the genes
linked genes close together are tightly linked
linked genes far apart on the same chromosome almost behave as unlinked genes
if genes are on same chromosome (linked) will not have equal ratio expectance as the closer together genes will be expected more
