Topic 7A - Genetics DVY * Flashcards
Inheritance Linkage and Epistasis The Chi-squared test
what is a gene?
a sequence of bases on a DNA molecule that codes for a protein which results in a characteristic
what is an allele?
a different version of a gene. most plants and animals carry 2 alleles of each gene. the order of bases in each allele so they code for different versions of the same characteristic
what is a genotype?
the genetic constitution of an organism - the alleles an organism has
what is a phenotype?
the characteristics of the genotype that are expressed, and its interaction with the environment
what is dominant?
an allele who’s characteristics always appear in the phenotype. even if there’s only 1 copy
what is recessive?
an allele whose characteristics only appear in the phonotype if it is homozygous
what is codominant?
alleles that are both expressed in the phenotype - neither are recessive
what is the locus?
the fixed position of a gene on a chromosome. alleles of a gene are found at the same locus on each chromosome in a pair
what is a homozygote?
an organism that carries 2 copies of the same allele
what is a heterozygote?
an organism that carries 2 different alleles
what is a carrier?
a person carrying an allele which is not expressed in the phenotype but that can be passed on to offspring
what is monohybrid inheritance?
the inheritance of a characteristic controlled by a single gene
e.g. N - normal wings
n- small wings
what is sickle cell anaemia?
SS - homozygous for sickle cell - red blood cells are misshapen so they don’t carry oxygen properly
NN - homozygous for normal haemoglobin - don’t have disease
SN - in between, patients can still survive but the haemoglobin is a strange shape - but mosquitos can’t pass on malaria
what is a dihybrid cross?
when the parents have 2 different genes you can work out the chances of offspring inheriting certain combinations of alleles.
e.g. genotype: RrYy
gametes: RY, Ry, rY, ry
these would go on the top of the punnet square and the other parents can go on the side
what is the phenotypic ratio?
the ratio of different phenotypes in offspring if both parents are heterozygous: monohybrid - 3:1 dihybrid - 9:3:3:1 codominant - 1:2:1
when is a characteristic sex-linked?
when the allele that codes for it is located on a sex chromosome
what are most sex linked genes carried on?
the Y chromosome is smaller than the X and carries fewer genes. so most genes are only carried on the X chromosome. (X-linked genes)
why are men more likely to express recessive phenotypes for sex-linked genes?
males only have 1 X chromosome, so only have 1 allele for X-linked genes.
so they express the characteristic of this allele even if it’s recessive
what are examples of genetic disorders caused by faulty alleles on sex chromosomes?
colour blindness
haemophilia
the faulty alleles are carried on the X-chromosome, so are more common in men than women
what is an autosome?
any chromosome that isn’t a sex chromosome
what are autosomal genes?
genes located on the autosomes
what is autosomal linkage?
genes on the same autosome are said to be linked, they stay together during independent segregation of chromosomes in meiosis I, and their alleles will be passed on to the offspring together.
this only doesn’t happen if crossing over splits them up
how can autosomal linkage be identified?
you don’t get the phonotypic ratio you expect in the offspring.
the linked alleles are inherited together, so a higher proportion of the offspring will have their parents genotype and phenotype.
if you do a cross and the ratios you find don’t match those provided, there is autosomal linkage
what is epistasis?
when the allele of 1 gene masks the expression of the alleles of other genes
what do epistatic genes do?
the phenotypic ratios are different to how expected, if a certain (epistatic) allele is present then other alleles are not expressed and so aren’t counted in he ratio
what is the chi-squared test?
a statistical test that’s used to see if the results of an experiment support a theory
how to carry out a chi-squared test?
make a null hypothesis - no significant difference between observed and expected
use the theory to predict the expected result, compare to observed results
carry out test and compare, the outcome will support or reject null hypothesis
what is the Chi-squared formula?
sum of (O-E)^2/E
how to find if there’s a significant difference between O and E?
compare X^2 value to critical value
if X^2 > critical value = significant difference
X^2 < critical value = no significant difference, difference due to chance
how are genes inherited?
genes are inherited by both parents, in sexual reproduction by fusion of gametes
2 copies of each gene are inherited, 1 dominant, 1 recessive
What kind of alleles are blood types?
A and B are codominant
O is recessive
only 2 alleles may be present in the loci
what is haemophilia?
when a mutated allele results in a non-functional protein used in the clotting process
slow persistent internal bleeding
it can be treated by giving patient the normal protein
