Genetics

Question 1

What is a gene?

Answer 1

Sequence of bases on a DNA molecule that codes for a protein (polypeptide) which results in a characteristic

Question 2

Alleles

Answer 2

• Different versions of the same gene
• There can be many different alleles of a single gene, but most plants and animals, including humans, only carry 2 alleles of each gene, one from each parent
• Order of bases in each allele is slightly different-that’s because each allele codes for different versions of the same characteristic (alleles represented using letters)

Question 3

Loci

Answer 3

• Humans are diploid organisms
• This means we have 2 copies of each chromosome-one from each parent
• It’s why we have 2 alleles of each gene
• The allele of each gene is found at a fixed position, called a locus, on each chromosome in a pair

Question 4

Genotype

Answer 4

• Genetic constitution/different alleles an organism has
• This could be a list of all its alleles but usually it’s just the alleles for one characteristic at a time

Question 5

Phenotype

Answer 5

• Expression of the genetic constitution and its interaction with the environment
• This just means what characteristics and organism has as a result of both its genes and the effect the environment has on its genes

Question 6

Dominant alleles

Answer 6

Always expressed in the phenotype, even when there’s only one copy of it (shown by capital letter)

Question 7

Recessive alleles

Answer 7

Those with characteristics that only appear in the phenotype if 2 copies are present (shown by lower case letters)

Question 8

Codominant alleles

Answer 8

Some alleles are both expressed in the phenotype beacuse neither one is recessive (blend of features or both features represented)

Question 9

Homozygous and Heterozygous

Answer 9

• At each locus in a diploid organism, the genotype can be homozygous or heterozygous
• If an organism carries 2 copies of the same allele, it’s said to be homozygous at that locus
• If an organism carries 2 different alleles for a gene, then it’s heterozygous
• An organism can be heterozygous at one locus and homozygous at another

Question 10

What are genetic diagrams?

Answer 10

• Diploid organisms have 2 alleles for each gene
• Gametes (sex cells) contain only one allele for each gene-they’re haploid
• When haploid gametes from 2 parents fuse together, the alleles they contain form the genotype of the diploid offspring that is produced
• Genetic diagrams are used to predict the genotypes and phenotypes of the offspring produced if 2 parents are crossed (bred)

Question 11

Monohybrid Inheritance

Answer 11

• Inheritance of a characteristic controlled by a single gene
• Monohybrid crosses show the likelihood of the different alleles of that gene (and so many different versions of the characteristic) being inherited by offspring of certain parents
• 2 homozygous parents will always produce all heterozygous in F1 generation

Question 12

Phenotypic ratios

Answer 12

• Ratio of different phenotypes in offspring
• Genetic diagrams allow you to predict the phenotypic ratios in F1 and F2 offspring
• Usually get 3:1 ratio of dominant:recessive characteristics with 2 heterozygous parents in monohybrid cross
• However, codominant alleles and sex linkage can alter this
• It is due to chance which gametes fuse with which

Question 13

What are charcteristics determined by in dipolid organisms?

Answer 13

Alleles that occur in pairs (only one of each pair of alleles can be present in a single gamete)

Question 14

Why is a large sample an advantage?

Answer 14

More likely the actual results come near to matching theoretical ones (large numbers-representative)

Question 15

Punnet Square

Answer 15

Another way of showing a genetic diagram

Question 16

Monohybrid inheritance of codominant alleles

Answer 16

• Example in humans is the allele for sickle-cell anaemia, a genetic disorder caused by a mutation in the haemoglobin gene (causes red blood cells to be sickle-shaped)
• Show main gene as a capital and alleles in small becauce neither is recessive
• Whenever you do a monohybrid cross with 2 heterozygous parents involving codominant alleles, you would expect a 1:2:1 ratio

Question 17

Multiple allele crosses

Answer 17

• Inheritance is more complicated when there are more than 2 alleles of the same gene (multiple alleles)
• There are 3 alleles associated with gene I (immunoglobulin) which lead to presence of different antigens on cell surface membrane of red blood cells
• Allele Io is recessive
• Alleles Ia and Ib are codominant- people with those alleles combined will be blood group AB
• Expected ratio is 1:1:1:1

Question 18

Dihybrid crosses

Answer 18

• Dihybrid inheritance is the inheritance of 2 characteristics, which is controlled by 2 different genes located on different chromosomes (each gene will have different alleles)
• Used to show likelihood of offspring inheriting certain combinations of the 2 characteristics from particular parents
• Dihybrid cross with 2 heterozygous parents is usually a 9:3:3:1 ratio (won’t always get this if epistasis or linkage are involved)
• Dihybrid cross between homozygous dominant and homozygous recessive will produce all heterozygous in F1
• Fertilisation is random during meiosis

Question 19

Law of independent assortment

Answer 19

Each member of a pair of alleles may combine randomly with either of another pair

Question 20

Dihybrid crosses and codominance

Answer 20

Similar to dihybrid crosses but there are more than 4 possible phenotypes in the offspring

Question 21

Inheritance of sex-linked characteristics

Answer 21

• Genetic information for gender is carried on 2 sex chromosomes
• In mammals, females have XX and males have XY (male/female is 50:50 chance)
• Some characteristics are sex-linked
• That means the alleles that code for them are located on a sex chromosome
• The Y chromosome is smaller than the X chromosome and carries fewer genes
• So most genes on the sex chromosomes are only carried on the X chromosome (X-linked genes)
• As males only have 1 X chromosome they often only have one allele for sex-linked genes
• So because they only have one copy, they express the characteristic of this allele even if it’s recessive
• This makes males more likely than females to show recessive phenotypes for genes that are sex-linked
• Genetic disorders caused by faulty alleles located on sex chromosomes include colour blindness and haemophilia
• The faulty alleles for both of these disorders are carried on the X chromosome and so are called X-linked disorders
• Y-linked disorders exist but are less common (colour blindness rare in women)

Question 22

Carrier

Answer 22

Person carrying an allele which is not expressed in the phenotype but that can be passed on to offspring

Question 23

Sex-linked cross

Answer 23

• Males can’t be carriers of X-linked disorders as they only have one copy of the chromosome
• 3:1 ratio of offspring
• But when a female carrier and a male without colour-blindness have children, only their male offspring are at risk of being colour blind (can also say there’s a predicted 2:1:1 ratio of female without colour blindness, male without colour blindness and male with colour blindness)
• Ratio will change if female carrier and male with colour blindness have children (1:1 with and without colour blindness)
• Ratio will be the same for offspring of each gender
• You only end up with this predicted ratio for a monohybrid F2 cross with a sex-linked characteristic

Question 24

Linkage of autosomal genes

Answer 24

• Autosome is the fancy name for any chromosome that isn’t a sex chromosome
• Autosomal genes are the genes located on the autosomes
• 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 I, and their alleles will be passed on to the offspring together
• The only reason this won’t happen is if crossing over splits them up first
• The closer together 2 genes are on the autosome, the more likely they are said to be linked
• This is because crossing over is less likely to split them up
• If 2 genes are autosomally linked, you won’t get the phenotypic ratio you expect
• In a dihybrid cross between 2 heterozygous parents you’d expect 9:3:3:1
• Instead expected to be 3:1 (monohybrid between 2 heterozygous) because 2 autosomally-linked alleles are inherited together
• Means that a high proportion of offspring will have heterozygous parents’ genotype and phenotype

Question 25

Autosomal Linkage

Answer 25

As males pass Y to sons, can't pass disease but can pass to daughter's via X chromosome

Question 26

What is epistasis?

Answer 26

* Many different genes can control the **same characteristic**-they interact to form the phenotype * This can be because the allele of one gene **masks** the **expression** of the alleles of other genes * Epistatic genes are usually at different loci

Question 27

Enzyme pathway

Answer 27

Question 28

Phenotypic ratios for epistatic genes

Answer 28

* Crosses involving epistatic genes won't always result in expected ratios * The phenotypic ratio you would expect to get from a dihybrid cross involving an epistatic allele depends on whether the epistatic allele is recessive or dominant

Question 29

Recessive epistatic alleles

Answer 29

If the epistatic allele is recessive than 2 copies of it will mask the expression of the other gene

Question 30

Dominant epistatic alleles

Answer 30

If the epistatic allele is dominant, then having at least one copy of it will block the expression of the other gene