7a (genetics)

Question 1

what is a gene?

Answer 1

sequence of bases on a DNA molecule that codes for a protein which results in a characteristic

Question 2

what is an allele?
how many do we have?
what is the difference between alleles?

Answer 2

versions of a type of gene.
most plants and animals carry 2 of each (one from each parent)
order of bases in each allele is slightly different

Question 3

what is a loci?

Answer 3

the fixed position on a gene at which an allele is found

Question 4

what is a genotype?

Answer 4

an organisms genetic constitution eg someone could have the genotype BB or Bb for eye colour

Question 5

what is the phenotype of an organism?

Answer 5

the expression of the genetic constitution and its interaction with the environment

Question 6

what is the dominant allele?
what is the recessive allele?

Answer 6

dominant is always expressed in the phenotype even if theres only one copy
recessive alleles are characteristics which only appear in phenotype if two copies are present.

Question 7

what are codominant alleles?

Answer 7

when both alleles are expressed in the phenotype because neither is recessive
eg horses can have white or coloured hair allele but neither is recessive so horses with a copy of each have a coat with both hairs

Question 8

what is homozygous/heterozygous?

Answer 8

homozygous- organism carries 2 copies of same allele
heterozygous- organism carries 2 different alleles for a gene

Question 9

how many alleles for each gene do diploid organisms have at each locus?
how many alleles for each gene do haploid organisms have at each locus?

Answer 9

2
1

Question 10

what is a genetic diagram?

Answer 10

a diagram that can be used to predict the phenotype and genotype of offspring if two parents are crossed

Question 11

what is monohybrid inheritance?

Answer 11

the inheritance of a characteristic controlled by a single gene. monohybrid crosses show the likelihood of the different alleles of that gene being inherited by offspring of certain parents

Question 12

what is a phenotypic ratio?

Answer 12

ratio of diff phenotypes in the offspring

Question 13

what are the F1 and F2 generations?

Answer 13

F1 is first set of offspring and F2 is second set

Question 14

what is the typical phenotypic ratio for a monohybrid cross of 2 heterozygous parents?

Answer 14

3:1 of dominant:recessive

Question 15

what is codominance?
eg?

Answer 15

both alleles are expressed in the phenotype as neither one is recessive
eg sickle-cell anemia

Question 16

what is the expected phenotypic ratio for a monohybrid cross of 2 heterozygous parents involving codominant alleles?

Answer 16

1:2:1

Question 17

what is a multiple allele cross?

Answer 17

still only involve 1 gene but the gene can have more than 2 alleles

Question 18

what is dihybrid inheritance?
what is a dihybrid cross?

Answer 18

inheritance of 2 characteristics which are controlled by different genes
dihybrid crosses show the likelihood of offspring inheriting certain combos of the 2 characteristics from particular parents

Question 19

what is the phenotypic ratio for a dihybrid cross with 2 heterozygous parents?

Answer 19

9:3:3:1 (dominant both:dominant 1 recessive 2:recessive 1 dominant 2:recessive both)

Question 20

why is the phenotypic ratio for dihybrid crosses and codominance not 9:3:3:1?

Answer 20

there are more than 4 poss phenotypes in the offspring

Question 21

what are male sex chromosomes?
female?

Answer 21

XY
XX

Question 22

what is sex linkage?

Answer 22

when an allele that codes for a characteristic is located on a sex chromosome

Question 23

why are most genes on sex chromosomes on the X?

Answer 23

because the Y is smaller so carries fewer genes

Question 24

why are males more likely than females to show recessive phenotypes for genes that are sex linked?

Answer 24

males only have 1 X so often only have 1 allele for sex linkage. so because they only have 1 copy, they express characteristic even if its recessive

Question 25

examples of X-linked disorders

Answer 25

colour blindness and haemophilia

Question 26

what is an autosome?
what is an autosomal gene?

Answer 26

a chromosome that isnt a sex chromosome
autosomal genes are genes located on autosomes

Question 27

why are genes on the same autosome said to be linked?
what does it mean when 2 genes are more closely linked?

Answer 27

because they stay together during independent segregation in meiosis 1 and their alleles are passed on to offspring together
theyre closer together on an autosome so less likely to be split up by crossing over

Question 28

what is a back cross?

Answer 28

crossing the offspring with one of the parents

Question 29

what is epistasis?

Answer 29

the interaction of different genes where the gene at one locus masks or supresses the expression of another gene at a second locus

Question 30

what is recessive epistasis?
what is the phenotypic ratio of crossing homozygous dominant w homozygous recessive?

Answer 30

2 recessive alleles at one gene locus will mask the expression of alleles at a 2nd gene locus
9:3:4 in the F2 gen

Question 31

what is dominant epistasis?
what is the phenotypic ratio of crossing homozygous dominant w homozygous recessive?

Answer 31

a dominant allele at one gene locus will mask the expression of alleles at a 2nd gene locus
12:3:1 in the F2 gen

Question 32

what is the chi² test?

Answer 32

statistical test to see if results of an experiment support a theory
need to make null hypothesis that states there is no sig diff between observed and expected results

Used when categorical data

Question 33

how do you work out chi² value?

Answer 33

Χ^2 = Σ x (O-E)^2 / E
Χ2 is the chi-square test statistic
Σ is the sum of
O is the observed frequency
E is the expected frequency

Question 34

what is the critical value? how do you work it out?

Answer 34

the value of X^2 that corresponds to a 0.05 level of probability that the difference between the observed and expected results is due to chance
1. work out degree of freedom (no of phenotypes-1)
2.find critical value corresponding to the degrees of freedom and probability level of 0.05 in the table

Question 35

how do you compare the X² value to the critical value?

Answer 35

if X^2 is larger than or equal to critical value, then there is sig diff between observed and expected so reject null as difference is not due to chance. vice versa