Inheritance - A13

Question 1

Define gene

Answer 1

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

Question 2

What are alleles?

Answer 2

-different versions of the same gene
-there can be many different alleles for a single gene but most plants and animals only carry two alleles of each gene - one from each parent
-the order of bases in each allele is slightly different because each allele codes for different versions of the same characteristic

Question 3

Define locus

Answer 3

the fixed position of the allele of each gene

Question 4

Define genotype

Answer 4

an organisms genetic constitution

Question 5

Define phenotype

Answer 5

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

Question 6

What is a dominant allele?

Answer 6

if presented, it is always expressed in the phenotype

Question 7

What is a recessive allele?

Answer 7

only expressed in the phenotype if two copies of the allele are present

Question 8

What are codominant alleles?

Answer 8

both alleles are expressed in the phenotype because neither one is recessive

Question 9

What are genetic diagrams used for?

Answer 9

predict the genotypes and phenotypes of offspring

Question 10

What is monohybrid inheritance?

Answer 10

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

Question 11

Why may you not get the expected phenotypic ratio?

Answer 11

random fusion of gametes, small sample size, lethal allele/genotype, codominant alleles, and sex linkage

Question 12

What would the ratio for monohybrid inheritance of codominant alleles look like?

Answer 12

-due to the codominance of alleles, two heterozygous parents would not produce a 3:1 ratio
-with heterozygous and codominance, you would expect to see a 1:2:1 ratio

Question 13

What is dihybrid inheritance?

Answer 13

The inheritance of two characteristics, which are controlled by different genes.
Each of the two genes will have different alleles.

Question 14

What does a dihybrid cross show?

Answer 14

The likelihood of offspring inheriting certain combinations of the two characteristics.

Question 15

What is the expected result from a dihybrid cross?

Answer 15

9:3:3:1
9 dominant both
3 dominant first, recessive second
3 recessive first, dominant second
1 recessive both

Question 16

Dihybrid and codominance(ratios)…

Answer 16

-works in the same way, but the phenotypic ratios produced are quite different(more than 4 possible phenotypes)
-even if neither genes have codominant alleles, you won’t always get a 9:3:3:1 ratio with 2 heterozygous parents
-This could be because of gene linkage/epistasis/ lethal genotypes/ small sample size/ random fusion of gametes

Question 17

Where is genetic information for sex carried?

Answer 17

on 2 sex chromosomes
-females have 2 X chromosomes, males have 1 X and 1 Y

Question 18

Sex-linked characteristics….

Answer 18

-the alleles that code for characteristics is located on the sex chromosomes
-the Y chromosome is smaller than the X, so carries fewer genes
-so most genes on the sex chromosome are only carried on the X chromosome(called X-linked genes)

Question 19

What is the impact of males only having 1 X chromosome?

Answer 19

-males only have 1 X chromosome so they only have one allele for sex-linked genes
-because they only have one copy, they express this even if it’s recessive
-this makes males more likely then females to show recessive phenotypes for sex-linked genes

Question 20

Why are X-linked disorders more common in men?

Answer 20

as they only need to inherit one recessive allele to express the disorder, whereas women would need to inherit two
-men can either be affected or unaffected, and women can be carriers

Question 21

What is an autosome?

Answer 21

Autosome is the name given to any chromosome that isn’t a sex chromosome

Question 22

What type of genes are said to be linked?

Answer 22

The alleles of different genes cannot assort independently of the genes are on the same chromosome. Such genes are said to be linked because they tend to be inherited together.

Question 23

What is linkage of autosomal genes?

Answer 23

-genes on the same autosome are said to be linked-that’s because they’ll stay together during the independent segregation of chromosomes in meiosis 1, and their alleles will be passed on to the offspring together
-this might not happen if crossing over splits them up first
-the closer together two genes are in the autosome, the more closely they are said to be linked-crossing over is likely to split them.

Question 24

What is a pedigree?

Answer 24

genetic inheritance is often shown in the form of a pedigree(family tree)

Question 25

For pedigrees, what does it mean if a child has a disease and parents do not?

Answer 25

-the disease is recessive -parents are heterozygous

Question 26

For pedigrees, what does it mean if a child does not have a disease, but parents do?

Answer 26

-disease is dominant -parents are heterozygous

Question 27

For pedigrees, how do sex-linked characteristics impact a child's phenotype?

Answer 27

-Daughter must receive father's X chromosome -if daughter has (recessive) disease, father must have it if it's sex-linked. -son must receive father's Y chromosome -can't describe father as heterozygous if it is sex-linked

Question 28

What is epistasis?

Answer 28

-many different genes can control the same characteristic - they interact to from the phenotype

Question 29

Define epistasis.

Answer 29

when an allele of one gene masks the expression of the alleles of other genes

Question 30

What are the phenotypic ratios for epistatic genes?

Answer 30

the ratio will depend on if the epistatic allele is dominant or recessive