6.1.2. Patterns of inheritance
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176 Terms
what is a heterosome referring to
a sex chromosome
what is the name of a non-sex chromosome
autosome
what is an autosome
a non-sex chromosome
what is a heterosome
a sex chromosome
how many autosomes do humans have
22 pairs
what is it called to have XX chromosomes
homogametic
what is it called to have XY chromosomes
heterogametic
what type of heterosomes do males have
heterogametic- XY, male birds are homogametic- XX
what type of heterosomes do females have
homogametic- XX, female birds are heterogametic- XY
what are homologous chromosomes
a pair of chromosomes, one from each parent, which are similar length, have same genes at the same loci- may have a different allele- and centromeres in the same position
what is co-dominance
a condition where two different alleles for a single gene are expressed in the phenotype
what is it called where two different alleles for a single gene are expressed in the phenotype
codominance
define diploid
where pairs of homologous chromosomes are present in the cell
define haploid
where only one copy of each chromosome is present in the cell
what must you do with gametes when writing them
draw a circle around them
what is the F1 generation
the generation produced from crossing 2 purebreed homozygous parental stocks, these are all heterozygous
what is the generation produced from crossing 2 purebreed homozygous parental stocks
F1 generation
what is the F2 generation
the generation produced from crossing 2 heterozygous F1 generations
what is the generation produced from crossing 2 heterozygous F1 generations
F2 generation
what is autosomal linkage
when two or more genes are located on the same chromosome
what is a monohybrid cross
single factor characteristic inheritance- only one gene being inherited, 1 pair of alleles on 1 pair of homologous chromosomes
what is the monohybrid ratio
ratio of phenotypes of F2 generation in a monohybrid cross: 3:1
what is the 3:1 ratio
monohybrid ratio (of F2 phenotypes)
what is the name of the ratio of phenotypes of F2 generation in a monohybrid cross
monohybrid ratio
what is the test cross/back cross
to ascertain whether an individual is heterozygous or homozygous dominant cross the individual with a homozygous recessive individual and if they are homozygous dominant all offspring will be heterozygous, if they are heterozygous some offspring will be recessive
what are the genetic mechanisms lead to genetic variation
genetic variation is variety of alleles
meiosis: alleles swapped in crossing over in prophase I, independent assortment in metaphase I and II
random fertilisation/fusion of gametes
random mating- panmixia
sexual selection (non-random mating)
internal fertilisation
external fertilisation
what is the ratio of phenotypes of the F2 generation with codominance
1:2:1- same ratio as genotypes
how do you write alleles for codominance/multiple alleles
large letter for characteristic and then superscript uppercase letter that is different for each version
what is multiple allele condition
when a single characteristic can have 3 or more forms- one gene has more than 2 alleles
what is it called when one gene can have more than 2 alleles
multiple alleles
if a gene has multiple alleles, how many does an individual have
the homologous chromosome will only contain 2 of these alleles at the locus of the gene (for a diploid individual)
what’s an example of a multiple alleles condition
human blood inheritance
what are the possible phenotypes of human blood
A, B, AB and O
what is the gene for human blood
I
how are alleles written for human blood
IA, IB, IO
what are the types of alleles for human blood- how are they related to each other
IA and IB are codominant- both express in phenotype, IO is recessive to both
why is blood group important
for matching blood for transfusions
which blood group is the universal donor
O- no antigens present on surface of RBCs
which blood group is the universal recipient
AB
what is sex-linkage
all genes carried on sex chromosomes
what are the genes carried on sex-chromosomes
sex-linked
what are the parts of XY chromosomes
homologous portion- gene/allele present on both X and Y, allows formation of bivalents+crossing over in prophase I
non-homologous portion- gene/allele only present on X chromosome
what happens when a characteristic is on the non-homologous portion of the X chromosome in a male
it will be expressed whether it is dominant or recessive
example of sex-linked condition
haemophilia- recessive condition, prevents formation of factor needed in clotting cascade, gene is present on non-homologous portion of X chromosome, females can only be carriers
how to write sex-linked gametes
XNXn for females or XNY for males
where do males get their phenotype for sex-linked genes from
their mother- their father passes on the Y chromosome
what is dihybrid cross
the inheritance of 2 characteristics/genes- 2 sets of alleles (4 alleles in total) on 2 pairs of homologous chromosomes (4 chromosomes in total), each gamete inherits 2 chromosomes
how to write genotypes for dihybrid crosses
keep alleles for same genes together i.e. RrGg not RGrg
how to write gametes for dihybrid crosses
both alleles in one gamete- need to include all possible combinations
what is the dihybrid ratio
F2 phenotypes of a dihybrid cross- 9:3:3:1
what is the 9:3:3:1 ratio
dihybrid ratio (of F2 phenotypes)
what is autosomal linkage of dihybrid cross
2 genes/ 2 pairs of alleles are on the same pair of homologous chromosomes
what needs to be considered with autosomal linkage
alleles are fixed on a chromosome so gametes can only be formed from the alleles on the same chromosome
what is the standard F2 phenotype ratio for autosomal linkage of dihybrid cross
3:1
why is the ratio for autosomal linkage of dihybrid cross often different to 3:1
crossing over in prophase I of meiosis affects ratio and creates recombinant chromosomes, allowing all 4 phenotypes in the offspring- when loci are close to each other or the centromere, fewer recombinant chromosomes are formed
what does crossing over in prophase I effect
the F2 phenotype ratio in autosomal linkage of dihybrid cross- changes from 3:1 to a possible mixture of all 4 phenotypes
what is the equation for recombination frequency
number of recombinant offspring/total number of offspring x100
what is recombination frequency used for
to map linked genes on a chromosome
what is used to map linked genes on a chromosome
recombination frequency
what is polygenic inheritance
when 2 or more genes at different loci can interact to affect one characteristic
what is it called when 2 or more genes at different loci can interact to affect one characteristic
polygenic inheritance
how are alleles inherited in polygenic inheritance
exactly same way as in dihybrid cross so same 9:3:3:1 ratio is produced in F2 generation, however only one characteristic is being looked at with a more diverse phenotype than a monohybrid cross
what is epistasis
two genes control a single characteristic but one of the genes can mask the effect of the other
what is it called when two genes control a single characteristic but one of the genes can mask the effect of the other
epistasis
what is the name of a gene that can mask the effect of another gene
an epistatic gene
what is an epistatic gene
a gene that can mask the effect of another gene
what is the name of a gene whose effect is masked by another gene
a hypostatic gene
what is a hypostatic gene
a gene whose effect is masked by another gene
example of epistasis
colour of labrador dogs
what are the types of epistasis
recessive and dominant
what is recessive epistasis
when the presence of 2 recessive alleles at one gene locus masks the expression of another gene
where does recessive epistasis commonly occur
between structural genes in a metabolic pathway, can be observed where the end-product is present in the phenotype
what is dominant epistasis
the presence of a dominant allele at a gene locus results in that gene masking the effect of another gene
example of dominant epistasis
the regulatory gene for the repressor protein which stops the expression of the structural genes in the Lac Operon
when is chi-squared used
determines the difference between observed and expected results e.g. in genetic crosses for the phenotype ratio
the difference between conditions in an experiment
how to conduct a chi-squared test when looking at genetic crosses
null hypothesis- no difference between observed and expected values
use known theoretical ratios to calculate the expected values for each category
subtract expected from observed value and square number
then divide by expected value
sum answers for each category
degrees of freedom= no. of classes-1
significant results if calculated value is greater than critical
how to calculate degrees of freedom for a chi-squared test
no. of classes-1
how to conduct a chi-squared test when used in an experiment
same as in genetic cross, but instead of calculating expected value using theoretical value, expected value is even numbers in all conditions
what happens to statistical significance in a chi-squared test as the sample size increases
likelihood of result being significant increases
define species
individuals who can breed with each other to produce fertile offspring
what are the forms of variables of characteristics in intraspecific variation
discontinuous and continuous variable
how many genes control discontinuous variation
one gene
what type of variation is controlled by one gene
discontinuous
how many genes control continuous variation
more than one gene- polygenic
what type of variation is controlled by more than one gene
continuous variation
effect of discontinuous variation on phenotype
feature is either present or absent, differences are in discrete, distinct categories with no intermediates- quantitative data
effect of continuous variation on phenotype
there are many intermediates between the extremes, features can be measured across a complete range- qualitative data
how does the environment affect discontinuous variation
very little influence
how does the environment affect continuous variation
has significant effect
what is panmixia
random mating, increases variation- all individuals of one sex are equally potential partners of all members of the opposite sex
what is sexual selection
non-random mating- probability that two individuals in a population will mate is not the same for all possible pairs of individuals- maintains fittest alleles
what is it called when all individuals of one sex are equally potential partners of all members of the other sex
panmixia- random mating
what is it called when one member of a population does all the matin
sexual selection- non-random mating
what is internal fertilisation
male gametes large in number and female gametes small in number
what is external fertilisation
male and female gametes large in number
what is phenotypic variation caused by
genetic mechanisms causing variation and environmental influences
what is it called when factors in the environment act to switch genes on or off
epigenetics
what is epigenetics
factors in the environment act to switch genes on or off
what is the advantage of inheriting epigenetics
next generation likely to be in similar environmental conditions so will be better adapted
what is the insufficient production of chlorophyll called
chlorosis