Lecture 18 (introduction to Mendelian genetics)

Question 1

Pea flower cross pollination

Answer 1

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

Question 2

Monohybrid cross

Answer 2

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.

Question 3

How can the results of no signs of blended inheritance in Mendel’s pea experiment be explained?

Answer 3

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

Question 4

Allele

Answer 4

Alternative forms of a gene

Question 5

Gene

Answer 5

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

Question 6

Dominance

Answer 6

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.

Question 7

Dominant allele

Answer 7

Dominant alleles being expressed over recessive alleles. Only one dominant allele is needed to show the dominant phenotype

Question 8

Recessive allele

Answer 8

Recessive alleles are only expressed when no dominant allele is present.

Question 9

How alleles and loci are related?

Answer 9

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.

Question 10

Genotype

Answer 10

Set of genes and/or DNA combinations that are responsible for a particular trait

Determined at a DNA level

Question 11

Phenotype

Answer 11

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)

Question 12

F1 generation

Answer 12

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.

Question 13

F2 generation

Answer 13

the F2 generation is the result of a cross between two F1 individuals (from F1 generation).

Question 14

Parental generation

Answer 14

The initial generation is given the letter “P” for parental generation.

Question 15

The genes for a particular trait …

Answer 15

Reside at a specific locus on a certain chromosome

Question 16

Punnett square

Answer 16

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

Question 17

heterozygous and homozygous

Answer 17

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.

Question 18

Phenotypes for recessive dominant and heterzygote ?

Answer 18

Both have the same phenotype (express the dominant phenotype)

Question 19

Monohybrid cross ratio in F2 generation
P generation - PP x pp
F1 generation - Pp x Pp

Answer 19

3:1 ratio in the F2 generation (can get the homozygous recessive phenotype back)

Question 20

Genotype and its link to phenotype

Answer 20

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.

Question 21

Test cross

Answer 21

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)

Question 22

Mendel’s laws on a chromosomal basis

Answer 22

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.

Question 23

Mendel’s first law

Answer 23

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

Question 24

Mendel’s second law

Answer 24

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

Question 25

Punnett square ratios

Answer 25

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

Question 26

Dihybrid cross

Answer 26

Dihybrid cross is a cross between two different lines/genes that differ in two observed traits.

Question 27

Dihybrid cross phenotypic ratio

Answer 27

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