Inheritance and Evolution U6

gentoype

gentoype

genetic constitution of an organism - refers to alleles present-BB/Bb-bb

phenotype

appearance of a characteristic due to the expression of the genotype and its interaction with the environment

alleles

different form of genes. can be dominant / recessive/ co dominant

dominant allele

always expressed in phenotype

recessive allele

only expressed when the genotype is homozygous recessive

codominant allele

both alleles are expressed in the phenotype- neither allele is dominant / equally dominant- with both contributing to the phenotype of the characteristic

locus

position of a gene on a chromosome. alleles occupy the same locus on each member of a pair of homologous chromosome.

in a diploid organism, the alleles at specific locus may be heterozygous or homozygous

homozygous

pair of homologous chromosomes carrying the same alleles for a single gene

heterozygous

pair of homologous chromosomes carrying two different alleles for a single gene

how are these types of inheritances presented via genetic diagram coding-

monohybrid

codominant

multiple alleles

sex- linkage

autosomal linkage

epistasis

monohybrid - single letter, capital/ lower case - B or b

codominant- Gene^ allele - I^B I^A etc

multiple alleles - Gene ^allele- I^B I^O etc

Sex - linkage - Chromosome^ allele- X^ R X^r / X^R Y

autosomal linkage - single letter, capital/ lower case - Aa Bb

epistasis- single letter, capital/ lower case EE Bb

multiple alleles

there are more than two alleles of a particular gene. however, only two alleles can be present in a genotype - one on each member of a pair of homologous chromosomes -

an example of this is ABO Blood Groups where the O is recessive- In one genotype there will be two alleles present in any one genotype- I^A = I^B these alleles are codominant so the blood group would be AB

sex linked gene

where the gene is carried on only one type of sex chromosome- usually X

autosomal linkage

genes that are licated on the same chromosome- not the sex chromosome

epistasis

when one gene modifies or masks the expression of a different gene at a different locus

what is monohybrid inheritance

inheritance of one particular characteristic- genetic inheritance cross of a characteristics determined by one gene

what is dihybrid inheritance?

inheritance of two different characteristics. each characteristic is controlled by a different gene and found on different chromosomes.

why is a test cross used?

to determine the genotype of an organism with a dominant phenotype. involves crossing the organism with the homozygous recessive

what does pure breed mean?

homozygous for each allele

sex-linked genes

the gene is carried on only one type of sex chromosome, usually the X (has more alleles in the extra tail that the Y chromosome does not have)

what are the non-sex chromosomes called

autosomes

what is colour blindness caused by

caused by a recessive allele carried on the X chromosome

the allele B for normal colour vision is dominant, allele b for colour blindness is recessive

the Y chromosome does not carry the allele

males cannot be heterozygous as they can carry only one allele on the X chromosome - X^BY (normal) or X^bY (colour blind)

what is haemophilia and what is it caused by?

draw a genetic cross diagram for a woman who is heterozygous and man who is “normal”

recessive condition in which the blood doesnt clot and sufferers can bleed to death if its not treated- it is sex linked and carried on the X chromosome

why are X linked genes guaranteed to he passed from mother to son?

male children must inherit their X chromosome from their mother- so if there are any x linked are guaranteed to be passed from mother to son

why can a man not pass a sex linked gene to his son?

males have a Y chromosome and this could only have been passed on from their father so the X chromosome must have come from their mother

why can a gene, if sex linked, be assumed to be X- linked?

there are no known examples of genes carried only on the Y chromosome

why does a recessive sex linked trait tend to occur most often in males?

there is no other allele on the Y chromosome, so the recessive allele is always expressed. but doesnt mean that females dont get disease- it is just less likely as they have to inherit two recessive alleles

how can you prove if the allele is recessive?

• 2 unaffected parents who have an affected child

• the parents musr therefore be heterozygous

how can you prove an allele is not sex linked (when known to be recessive)

• 2 unaffected parents have an affected daughter

• the father would pass on the dominant allele on the X chromosome so all daughters should be unaffected however there is a daughter with the allele therefore - not sex linked ( has to be recessive)

how can you prove an allele is dominant

both parents must be heterozygous and pass on their recessive alleles

when can you use the chi- squared test in relation to genes

to compare the goodness of fit of observed phenotypic ratios with expected ratios