Unit 7 - Genetics, Populations, Evolution and Ecosystems

what is a gene?

is a sequence of bases on a DNA molecule that codes for a protein (polypeptide)

what are alleles?

are different versions of genes that code for variants of a characteristic

what is a locus?

is a specific position of a gene on a chromosome

what is the difference between a dominant allele and a recessive allele?

a dominant allele is always expressed in the phenotype even when only one is present, whereas a recessive allele is only expressed when it is homozygous in the genotype

what is monohybrid inheritance?

inheritance of a single gene

how do you use a punnett square to perform a genetic cross?

state the phenotypes of the two parents
assign the genotypes of the parents letter codes
state the gametes of the parents
use the punnett square to show all possible results of the random fertilisation of parental gametes
state the proportion of each genotype among the offspring as a ratio or percentage
state the proportion of each phenotype among the offspring

what is the difference between a homozygous organsim and a heterozygous organism?

an organism with two identical alleles for a trait is homozygous
an organism with two different alleles for a trait is heterozygous

what is the difference between a genotype and a phenotype?

a genotype is a genetic makeup for a trait, a phenotype is the physical characteristics that the genotype

what is the F1 generation in genetic crosses?

consists of offspring from a cross between individuals with homozygous dominant/recessive genotypes this means they will therefore all express the dominant trait in their phenotypes

what is the F2 generation?

results from a cross between two F1 individuals exhibiting a 3:1 ratio of dominant:recessive phenotypes

what is a dihybrid cross?

shows the two simultaneous inheritance of two genes controlling separate characteristics

what are linked genes?

are located on the same chromosome and tend to be inherited together unless crossing over occurs 

how does the law of independent assortment apply to dihybrid crosses?

alleles for different genes segregate independently during gametes formation in meiosis

how do you denote the genotype of a dihybrid organism?

the genotype is written by placing the alleles for one gene immediately followed by the alleles fro the second gene

in genetics, what does a true breeding or pure breeding mean?

is homozygous fro the traits being studied, consistently producing the same trait in offspring

predict the genotype the phenotype of F1 offspring from a dihybrid cross of two truw breeding parents

all F1 offspring will have a heterozygous genotype, displaying both dominant traits in their phenotype

why might the actual offspring ratio differ from the expected in a dihybrid cross?

may differ from the expected offspring ratio in a dihybrid corss due to random fertilisation of gametes or if the genes are linked

how many types of gamete can be produced by an organism with a genotype that is heterozygous for both dihybrid genes?

a heterozygous dihybrid organism can produce four types of gamete

what ratio is typical of a dihybrid cross with both parents heterozygous at both gene loci?

the expected ratio is 9:3:3:1

what is codominance?

occurs when the 2 alleles are equally dominant, resulting in both being expressed in the phenotype

how is codominance different from complete dominance?

both alleles are expressed whereas in complete dominance only one allele is expressed

how does codominance affect the phenotype?

both alleles contribute to the phenotype, resulting in a mixture of blend of traits

how are codominant alleles represented?

are represented as different superscript uppercase letters to the right of an uppercase letter that represents the gene

state the genotypes of the offspring of a cross between a plant that produces white flowers C^WC^W and a plant that produces red flowers C^RC^R

the offspring will have the heterozygous genotype C^WC^R

state the phenotypes of the the offspring produced between a plant that produces white flowers C^WC^W and a plant that produces red flowers C^RC^{R given that the alleles for flower colour are codominant}

their phenotype would be a mix of red and white flowers so they would be pink flowers 

what are multiple alleles?

refer to a gene having more than 2 allele options. although only 2 can be present in an individual

why can an individual only have two alleles for each gene with multiple alleles?

they can usually still only have two alleles for each gene with multiple alleles because each person typically inherits only one allele from each parent

what is an example of multiple alleles in humans?

the ABO blood group system, determined by immunoglobulin gene which has 3 alleles, IA IB and IO

how many different blood group phenotypes are there and what are the genotypes that code for them?

blood group A - IAIO or IAIA 
blood group B - IBIO or IBIB
blood group AB - IAIB 
blood group O - IOIO

what antigens do the alleles IA, IB, IO each produce?

the IB allele leads to the production of antigen B
the IA allele leads to the production of antigen A
the IO allele does not lead to production of any antigens

which allele is recessive in the ABO blood group system?

IO is recessive to both IA and IB

which alleles are codominant in the ABO blood group system?

IA and IB are codominant alleles

what is sex linkage?

genes carried on the X or Y chromosome are said to be sex linked

what are the sex chromosomes in female human body cells and what sex chromosomes will their gametes contain?

have two X sex chromosomes their gametes can therefore only contain one X sex chromosome

what are the sex chromosomes in male human body cells and what sex chromosomes will their gametes contain?

male human body cells only contain one X chromosome and one differently shaped Y chromosome their gametes will contain with a X chromosome or a Y chromosome 

what is an X linked recessive disorder and why are they more common in males?

an X linked recessive diorder is one that is caused by the defective gene on the X chromosome, because males only have one X chromosome so the recessive allele is always expressed as it is never masked by a dominant allele

what is heamophilia?

is an X linked recessive disorder that cause blood to clot too slowly, leading to excessive bleeding

how do males inherit haemophilia?

males only inherit one X chromosome which always comes from their mother if they inherit it from their mother has the recessive haemophilia allele the male wont be able to produce clotting protein so will have haemophilia

what is a carrier for haemophilia?

a carrier for haemophlia is always a female who is heterozygous, this individual carriesthe recessive allele without showing symptoms of haemophilia as they can still prduce the clotting protein

can males pass haemophilia to their sons and why?

no they cannot this is because male pass the Y chromosome to their sons and the haemophilia allele is always present on an X chromosome

can males pass haemophilia to their daughters and why?

yes because females have two X chromosomes so males and females can pass the X chromosome to their daughters

what is a pedigree chart?

is a diagram that shows the occurrence and inheritance of a trait in a family across generations

what are autosomes?

are chromosomes that are not sex chromosomes 

what does it mean when gene are linked?

are located on the same chromosome and tend to be inherited together 

how does crossing over effect linked genes?

crossing over during meiosis can separate linked genes leading to genetic recombination and producing recombinant offspring

what is the role of chiasmata in linkage?

are regions where crossing over occurs, potentially separating linked genes

why are closely located genes more likely to be linked?

genes closer together are more likely to be linked as they have a lower chance of being separated by crossing over, closer genes therefore produce fewer recombinants as they’re less likely to be separated

can lined genes assort independently?

no lilnked genes do not undergo independent assortment unless they are separated by crossing over

what is epistasis?

when the allele of one gene affects the expression of another gene in the phenotype

what is an epistatic gene?

is one that affects the expression of another gene

what is recessive epistasis?

occurs when two recessive alleles affect the expression of another genew

what is dominant epistasis?

occurs when one or two dominant alleles can affect the expression of another gene

how does epistasis affect biochemical pathways?

if a gene in a pathway is prevented from being expressed by epistasis, subsequent reactions are blocked, masking other genes

if gene 1 produces blue pigment when a dominant allele B is in the genotype and gene 2 makes the blue print green when a dominant allele G is in the genotype what would be the phenotype if the genotype was bbGG?

the genotype bb means no blue pigment is made so no pigment is made at all don’t matter on the second gene