Lecture 18 (introduction to Mendelian genetics) Flashcards
Pea flower cross pollination
Can accurately perform cross pollination on peas. This process is a way of preventing self pollination so that the genotypes and phenotypes of the pea plants can be studied
Offspring of this experiment show that purple is dominant and that the flower colour trait is heritable
Monohybrid cross
A monohybrid cross is a breeding experiment between P generation (parental generation) organisms that differ in a single given trait. The P generation organisms are homozygous for the given trait. However, each parent possesses different alleles for that particular trait.
How can the results of no signs of blended inheritance in Mendel’s pea experiment be explained?
Results can be explained if… Information passed on from parents to offspring is packaged as discrete particles. Mendel called these factors. We now call them GENES. Also it appears that individuals have two copies of each gene, one inherited from the mother and the other from the father. Genes controlling a trait can differ - these different forms of a gene are called ALLELES
Allele
Alternative forms of a gene
Gene
A defined region (sequence) of DNA that produces a type of RNA molecules that has some function
Fundamental unit of inheritance
Codes for a protein
Dominance
dominance is the phenomenon of one variant of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome.
Dominant allele
Dominant alleles being expressed over recessive alleles. Only one dominant allele is needed to show the dominant phenotype
Recessive allele
Recessive alleles are only expressed when no dominant allele is present.
How alleles and loci are related?
An allele is a variant form of a gene. Some genes have a variety of different forms, which are located at the same position, or genetic locus, on a chromosome.
Genotype
Set of genes and/or DNA combinations that are responsible for a particular trait
Determined at a DNA level
Phenotype
The physical expression, or characteristics of a genotype i.e. are traits or characteristics of an organism that can be observed
Defined by what happens at a protein level (together with environmental influences)
F1 generation
The F1 generation refers to the first filial generation. Filial generations are the nomenclature given to subsequent sets of offspring from controlled or observed reproduction.
F2 generation
the F2 generation is the result of a cross between two F1 individuals (from F1 generation).
Parental generation
The initial generation is given the letter “P” for parental generation.
The genes for a particular trait …
Reside at a specific locus on a certain chromosome
Punnett square
The Punnett square is a square diagram that is used to predict the genotypes of a particular cross or breeding experiment.
On the outside of the punnett square you put the alleles in the male and female gametes and the genotype inside the squares are probable genotypes for the offspring of the male and female that have mated together
heterozygous and homozygous
Genes come in pairs, called alleles, and each pair is located in a specific position (or locus) on a chromosome. If the two alleles at a locus are identical to each other, they are homozygous; if they are different from one another, they are heterozygous.
Phenotypes for recessive dominant and heterzygote ?
Both have the same phenotype (express the dominant phenotype)
Monohybrid cross ratio in F2 generation
P generation - PP x pp
F1 generation - Pp x Pp
3:1 ratio in the F2 generation (can get the homozygous recessive phenotype back)
Genotype and its link to phenotype
The genotype of an organism is defined as the sum of all its genes. The phenotype of an organism is the observable physical or biochemical characteristics of an organism, determined by both genetic make-up and environmental influences.
Test cross
To identify whether an organism exhibiting a dominant trait is homozygous or heterozygous for a specific allele, a scientist can perform a test cross. The organism in question is crossed with an organism that is homozygous for the recessive trait, and the offspring of the test cross are examined.
If the dominant individual is homozygous dominant then all will show the dominant phenotype (100%)
If the dominant individual is heterozygous then the offspring will exhibit a 1:1 ratio between the recessive and dominant phenotypes (50/50)
Mendel’s laws on a chromosomal basis
Both of these laws are explained by chromosomal behaviour during meiosis
Genes lie on chromosomes and alleles are alternative forms of a gene.
Chromosomes, like Mendel’s genes, come in matched (homologous) pairs in an organism. For both genes and chromosomes, one member of the pair comes from the mother and one from the father.
The members of a homologous pair separate in meiosis, so each sperm or egg receives just one member. This process mirrors segregation of alleles into gametes in Mendel’s law of segregation.
The members of different chromosome pairs are sorted into gametes independently of one another in meiosis, just like the alleles of different genes in Mendel’s law of independent assortment.
Mendel’s first law
Mendel’s first law is also known as the law of segregation. The law of segregation states that, ‘the alleles of a given locus segregate into separate gametes. ‘ Alleles sort independently because the gene is located on a specific chromosome.
“genes segregate at meiosis so that each gamete contains only one of the two possessed by the parent”
Following meiosis 2, each gamete has just one unreplicated copy of each chromosome and thus one allele per pair of genes
Mendel’s second law
Mendel’s second law is also known as the law of independent assortment. The law of independent assortment states that the alleles of one gene sort into gametes independently of the alleles of another gene.
“Alleles of different genes assort independently during gamete formation”
As a (heterozygote cell for both alleles i.e. RrYy) cell reaches Metaphase 1 of meiosis, the paired homologs can line up one of two ways
Punnett square ratios
Two heterozygous individuals breed together - 3 dominant phenotypes (2 heterozygotes and one homozygous dominant) : 1 recessive phenotype
Homozygous dominant and homozygous recessive - all offspring are heterozygotes
Dihybrid cross
Dihybrid cross is a cross between two different lines/genes that differ in two observed traits.
Dihybrid cross phenotypic ratio
When a RRYY (homozygous dominant for both alleles) and a rryy (homozygous recessive for both alleles) breed together the phenotypic ration is 100% homozygotes
9:3:3:1 when two heterozygotes breed
