Genetics Flashcards
1
Q
What is a gene?
A
Sequence of bases on a DNA molecule that codes for a protein (polypeptide) which results in a characteristic
2
Q
Alleles
A
- 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)
3
Q
Loci
A
- 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
4
Q
Genotype
A
- 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
5
Q
Phenotype
A
- 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
6
Q
Dominant alleles
A
Always expressed in the phenotype, even when there’s only one copy of it (shown by capital letter)
7
Q
Recessive alleles
A
Those with characteristics that only appear in the phenotype if 2 copies are present (shown by lower case letters)
8
Q
Codominant alleles
A
Some alleles are both expressed in the phenotype beacuse neither one is recessive (blend of features or both features represented)
9
Q
Homozygous and Heterozygous
A
- 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
10
Q
What are genetic diagrams?
A
- 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)
11
Q
Monohybrid Inheritance
A
- 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
12
Q
Phenotypic ratios
A
- 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
13
Q
What are charcteristics determined by in dipolid organisms?
A
Alleles that occur in pairs (only one of each pair of alleles can be present in a single gamete)
14
Q
Why is a large sample an advantage?
A
More likely the actual results come near to matching theoretical ones (large numbers-representative)
15
Q
Punnet Square
A
Another way of showing a genetic diagram
16
Q
Monohybrid inheritance of codominant alleles
A
- 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
17
Q
Multiple allele crosses
A
- 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
18
Q
Dihybrid crosses
A
- 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
21
Q
Inheritance of sex-linked characteristics
A
- 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)
23
Q
Sex-linked cross
A
- 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
24
Q
Linkage of autosomal genes
A
- 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
25
Q
Autosomal Linkage
A
As males pass Y to sons, can’t pass disease but can pass to daughter’s via X chromosome
27
Q
Enzyme pathway
A
29
Q
Recessive epistatic alleles
A
If the epistatic allele is recessive than 2 copies of it will mask the expression of the other gene
30
Q
Dominant epistatic alleles
A
If the epistatic allele is dominant, then having at least one copy of it will block the expression of the other gene
