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
what is blood group O?
IOIO, anti A and Anti B antibodies, no antigens on RBC
how are positive/negative blood groups determined?
if you have the rhesus (Rh) D antigen your blood type is positive, if not you are negative
what’s the difference between chromatids and chromosomes?
Before replication, one chromosome is composed of one DNA molecule, following replication, each chromosome is composed of two identical DNA molecules (i.e. DNA replication increases the amount of DNA but does not increase the number of chromosomes), the two identical copies - each forming one half of the replicated chromosome - are called chromatids, during the later stages of cell division these chromatids separate longitudinally to become individual chromosomes
what is the human karyotype?
46 chromosomes, 23 pairs, 22 of the 23 pairs have homologous partners which are identical, 23rd pair are sex chromosomes (X and Y)
what is the male genotype?
XY
what is the female genotype?
XX
what is the male/female offspring ratio?
1:1
what is sex linkage?
genes found on either X or Y are sex-linked, X is longer than Y, so some genes found on X do not have a homologous equivalent on Y, recessive characteristics characteristics found on this portion more frequent in men because no homologous section on Y that could carry the dominant allele
what is haemophilia?
x linked genetic disorder (defective gene on x), blood doesn’t clot correctly, excess bleeding or persistent internal bleeding, fatal if not treated, fatalities selectively reduced gene in populations to approx. 1 in 20,000, extremely rare in females because have 2 x chromosomes, haemophiliac females used to die at the onset of puberty, recessive, recessive allele alternative base sequence, faulty protein, healthy version of protein allows clotting, healthy protein produced by GM organisms
what are pedigree charts?
allow us to trace inheritance of sex-linked characteristics
what actually happens when you cross two individuals that are heterozygous for each characteristic? Why?
get a 3:1 ratio (grey normal:black vestigial), all of our genes are found on 23 pairs of chromosomes, this means individual chromosomes carry many genes, any 2 genes that are found on the same chromosome are linked, all genes on the same chromosome are linked
what is autosomal linkage?
when two or more genes are carried on the same autosome, all linked genes stay together during meiosis, this means they will pass into the gamete together, meaning they will pass into offspring together, they do not follow mendel’s law of independent assortment as each allele is not free to mix with either allele from another pair
what is Mendel’s law of Independent Assortment?
each member of a pair of alleles may combine randomly with either of another pair
which fly characteristics are linked?
grey body colour and normal wings GN, black body colour and vestigial wings gn
what is the expected ratio when crossing two heterozygotes for autosomal linkage?
3:1 (3 with dominant characteristic 1 with recessive)
what is the ratio when crossing 2 heterozygotes with monohybrid inheritance?
3:1
what is the ratio when crossing 2 heterozygotes with dihybrid inheritance?
9:3:3:1
what is the expected ratio when crossing blood group heterozygous A and B?
1:1:1:1
what is the expected ratio when crossing heterozygotes with codominant inheritance?
1:2:1
what is meant by epi?
on, upon, over (among, at, after, to, outside)
what is epistasis?
a condition whereby one gene controls the expression of another gene or when an allele of one gene effects or masks the expression of another in the phenotype
what is the example for epistasis?
fur of mice - gene A: distribution of melanin (A=banded a=uniform colour), gene B:coat colour by determining whether A is expressed B produces melanin b no melanin
what is agouti fur (epistasis)?
mousey coloured, wild type, grey/brown, banded fur with melanin genotype A_B_
what is albino (epistasis)?
no melanin ay all, white genotype A_/aa bb
what is black fur (epistasis)
uniform black hairs, no banding, has melanin aa B_
how does the expression of gene B (melanin) affect the expression of gene A (bands) with epistasis?
bb- no melanin/white, if there is no melanin then gene A cannot be expressed (no pigment to form the bands), this means that Aa, AA and aa will all produce white fur if bb is present, if B is present in the genotype, melanin is produced, if A and B are produced together, then melanin and banding are produced so agouti, if B is found with aa then melanin is produced but bands are not so black
what are biochemical pathways?
some genes act in a sequence by coding for specific enzymes in a pathway, dominant alleles code for the functional form of the enzyme, recessive homozygous would disrupt the pathway, the presence of one non-functional gene would affect the other as failure to express one gene will result in no pigment being made
What is the chi squared test?
Used when the data is categoric, comparing the observed to the expected value, to test the significance of differences between observed and expected results, measures the degree of deviation from O to E, comparing two or more data sets
What are the rules for chi squared?
Discrete categories, large sample size (20+), raw data, portion of numbers expected in each category is known
What are the steps to carry out chi squared?
Theory used to predict results (expected), experiment carried out (observed results), make a null hypothesis, observed result always a bit different, need to know if this is significant or due to chance
What is a null hypothesis?
Assumes there is no difference between observed and expected, chi squared used to reject or accept a null hypothesis e.g. there is no significant difference in lung cancer cases of smokers vs non smokers
What is the formula for chi squared?
Chi squared equals the sum of:(observed-expected) squared over expected
What are degrees of freedom?
Number of categories (n) - 1
What is the accepted confidence level for chi squared?
95% confidence level, 0.05 level of probability that the results are due to chance
What happens if you get a chi squared smaller than the critical value at 0.05 level of probability?
No significant difference between observed and expected, any difference due to chance, null hypothesis accepted
What happens if chi squared is larger than critical value at 0.05 level of probability?
Significant difference between observed and expected, statistically significant, something other than chance causing difference, null hypothesis rejected
