Genetics Flashcards
What is a gene?
Sequence of bases on a DNA molecule that codes for a protein (polypeptide) which results in a characteristic
Alleles
- 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)
Loci
- 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
Genotype
- 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
Phenotype
- 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
Dominant alleles
Always expressed in the phenotype, even when there’s only one copy of it (shown by capital letter)
Recessive alleles
Those with characteristics that only appear in the phenotype if 2 copies are present (shown by lower case letters)
Codominant alleles
Some alleles are both expressed in the phenotype beacuse neither one is recessive (blend of features or both features represented)
Homozygous and Heterozygous
- 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
What are genetic diagrams?
- 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)
Monohybrid Inheritance
- 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
Phenotypic ratios
- 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
What are charcteristics determined by in dipolid organisms?
Alleles that occur in pairs (only one of each pair of alleles can be present in a single gamete)
Why is a large sample an advantage?
More likely the actual results come near to matching theoretical ones (large numbers-representative)
Punnet Square
Another way of showing a genetic diagram
Monohybrid inheritance of codominant alleles
- 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
Multiple allele crosses
- 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
Dihybrid crosses
- 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
Law of independent assortment
Each member of a pair of alleles may combine randomly with either of another pair
Dihybrid crosses and codominance
Similar to dihybrid crosses but there are more than 4 possible phenotypes in the offspring
Inheritance of sex-linked characteristics
- 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)
Carrier
Person carrying an allele which is not expressed in the phenotype but that can be passed on to offspring
Sex-linked cross
- 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
Linkage of autosomal genes
- 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
Autosomal Linkage
As males pass Y to sons, can’t pass disease but can pass to daughter’s via X chromosome
What is epistasis?
- 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
Enzyme pathway
Phenotypic ratios for epistatic genes
- 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
Recessive epistatic alleles
If the epistatic allele is recessive than 2 copies of it will mask the expression of the other gene
Dominant epistatic alleles
If the epistatic allele is dominant, then having at least one copy of it will block the expression of the other gene
