inheritance Flashcards
what are the rules for genetic crosses?
choose a letter to represent the trait you are investigating, letters must be different lower case and capital, capital = dominant, lower case = recessive, label parents and their phenotype, label and circle gametes, draw a punnet square, label the gender of each parent, always write dominant allele first, work out the cross, state the phenotypes, calculate the ratio
what is a monohybrid cross?
a cross in which the alleles of only one gene are involved, inheritance of a single gene, used to determine the dominance relationship between alleles, cross begins with the parental (P1 or P) generation, e.g. pea pod colour (green and yellow)
what is the first step for monohybrid crosses?
determine the gametes from each parent
what is the second step for monohybrid crosses?
create punnet square
what is the third step for monohybrid crosses?
determine the phenotypic ratio
what is the expected ratio when crossing heterozygotes in monohybrid crosses?
3 with dominant trait 1 with recessive trait (75% 25%)
what is the basic law of genetics?
in diploid organisms, characteristics are determined by alleles that occur in pairs, only one of each pair of alleles can be present in a single gamete
how do you find out the probability of each gender offspring (monohybrid crosses)?
divide the probability by two
what are dihybrid crosses?
consider he inheritance of two characteristics at the same time, show how two characteristics determined by two different genes located on different chromosomes are inherited
what did Mendel observe (Mendel’s crosses of peas)?
seeds of his pea plants varied in two ways, seed colour (yellow and green) and seed shape (round and wrinkled), yellow and round were dominant and green and wrinkled were recessive
what are the four possible combinations of the two characteristics that Mendel observed?
round and yellow (RY), round and green (Ry), wrinkled and yellow (rY), wrinkled and green (ry)
what did Mendel find of the F1 generation when crossing homozygotes?
all RrYy, all would be heterozygous for both characteristics, all would ne round and yellow
what does the ratio always add up to in dihybrid crosses?
16
what is the typical ratio for a dihybrid cross?
9:3:3:1
what do the results of Mendel’s dihybrid crosses show us?
the F1 generation produce 4 types of gamete e.g. RY, ry, Ry, rY, they are able to do this as the gene for colour and the gene for shape are on four different chromosomes, during meiosis the chromosomes can arrange randomly at the equator, this means either shape allele can combine with either colour allele, fertilisation is random so any of the 4 gametes from one parent can mix with any four from the other parent
what is Mendel’s law of independent assortment?
each member of a pair of alleles may combine randomly with either of another pair
what is meant by codominance?
both alleles expressed in the phenotype, alleles are equally dominant, e.g. snapdragons; one allele codes for enzyme that catalyses synthesis of red pigment, other allele codes for altered version of enzyme that doesn’t produce pigment (white), homozygous for allele 1 = red, homozygous for allele 2 = white, heterozygous = only enough pigment for pink
what are the rules for codominance?
can’t use upper/lower case (implies dominant/recessive), use different letters instead (R=red, W=white), letters should be superscript to the letter that represents the gene C=colour
what is multiple alleles?
have more that 2 alternative forms of a single gene, located at the same loci of homozygous chromosomes, multiple alleles are involved in the determination of a single trait by codominance
what are polygenic traits?
determined by several genes at different gene locus, e.g. eye colour
what is the example for multiple alleles?
human blood group - 3 alleles associated with the immunoglobulin gene (I), gene I dictates the presence of certain antigens on the cell surface of red blood cells, 3 alleles, only two can be present, only two homologous chromosomes/gene loci, IA and IB are codominant IO is recessive to both, 4 possible blood groups
what is blood group A?
IAIA,IAIO, A antigens on RBC with anti-B antibodies in the plasma
what is blood group B?
IBIB,IBIO, B antigens on RBC Anti A antibodies in plasma
what is blood group AB?
IAIB, no antibodies to combat blood transfusions, A and B antigens on RBC