Lecture X: Genetics and Mendel

Question 1

What is a gene?

Answer 1

The basic unit of heredity
A gene is a section of DNA on a chromosome that carries the instructions to make a specific protein. This protein will have a structural or functional effect within an organism (a characteristic).
Some genes play a role in determining physical appearance, characteristics like eye colour, height, hair texture etc….

Question 2

Gene Locus

Answer 2

The exact location on the chromosome that the gene resides. The locus for a gene does not change. For example the locus for the flower colour gene will be on the same chromosome and in the same position for all plants of that species.

Question 3

Homologous chromosomes

Answer 3

Copies of the same chromosome are called homologous chromosomes because they share the same genes.
That said, They are not identical. They have the same genes, but may have variations in those genes.

Question 4

Allele

Answer 4

Homologous chromosomes are nopt identical, but different variations of genes are called alleles.
An allele are Different versions of the same gene. Each one will code for a trait of the same characteristic.

Question 5

Dominance of Alleles

Answer 5

We have 2 alleles for every gene because we have 2 copies of each chromosome. So what happens if the 2 alleles for a particular gene are not the same?

Dominant alleles for a gene are always expressed as a trait when present. Dominant alleles are always denoted with a capitol letter.
Recessive alleles for a gene are only expressed when an individual has 2 copies of it. Recessive alleles are denoted with a lower case letter.

Question 6

What is a Genotype?

Answer 6

The combination of alleles an individual has for a particular gene.
There are 3 possible genotypes for a gene that has only 2 alleles

e.g. If eye colour is denoted by the letter E. Brown eye colour is dominant (E) and blue eye colour is recessive (e).

Possible Genotypes: EE Ee ee

Question 7

What is Homozygous or Heterozygous

Answer 7

When an individual’s genotype has 2 alleles the same it is called homozygous. E.g. Genotypes EE or ee for eye colour.

When an individual’s genotype has 2 different alleles it is called heterozygous. E.g. Genotype Ee for eye colour.

Question 8

Phenotype vs Genotype

Answer 8

The phenotype for a gene is the trait that is expressed in the individual for a particular characteristic. E.g. Blue eyes or brown eyes are different traits for the characteristic of eye colour.

The genotype is the Letters (EE, Ee, ee)

Question 9

Whos is gregor mendel, what did he do?

Answer 9

Discovered how traits are passed on from one generation to the next.
Mendel experimented with pea plants. He chose plants with certain traits and cross bred them with plants with other traits. The phenotypes of the offspring were quantified and analyzed. E.g. Purple flowers X white flowers.

Question 10

What were the results of Mendel’s Model?

Answer 10

1. Alternative versions of genes account for variations in inherited characters (We call theses alleles).
2. For each character, an organism inherits 2 copies of a gene, one from each parent (alleles may be the same or different).
3. If the 2 alleles differ at a gene locus, then the dominant allele determines the individuals appearance (Phenotype) and the recessive allele has no noticeable affect on the individuals appearance.
4. Law of Segregation
5. Law of Independent Assortment
6. and 5. are part of the Laws of Heredity

Question 11

What is the law of segregation?

Answer 11

During gamete formation, the two alleles for a gene are separated when the homologous chromosomes move toward opposite ends of the cell and become placed in separate daughter cells during meiosis. Therefore, gametes will have one or the other allele for that gene.

Example of the Law of Segregation:
A person that is heterozygous for the gene determining freckles (genotype Ff). When meiosis occurs in this individual the Allele F (freckles) and Allele f (no freckles) are separated from each other and end up in different gametes.

Question 12

What is the law of Independent Assortment?

Answer 12

Alleles of genes for different heritable characters (e.g. Eye colour and hairline) separate independently of each other during gamete formation if they are on different chromosomes.

Law of Independent Assortment example:
Now we take into consideration 2 characteristics determined by genes on different chromosomes. In this example, the gene for freckles is on a different chromosome than the gene for hairline. This individual had inherited both dominant alleles from the mother (F and W) and recessive alleles (f and w) from the father. Random alignment of homologous pairs of chromosomes during metaphase I means that the alleles inherited from one parent don’t necessary stick together when chromosomes are divided up amongst gametes.

Question 13

What is the relevance of the laws of heredity?

Answer 13

It explains why traits sometimes skip generations.

Sometimes an individual carries a recessive allele, but does not express it in their phenotype, but some of their gametes will have that recessive allele.

Therefore it is possible for their children to express the recessive trait if their mate shares it too.

Question 14

What is a Monohybrid Cross?

Answer 14

When 2 individuals that are both heterozygous for a characteristic are crossed.
Pp X Pp

Question 15

What is a Punnet Square?

Answer 15

Determines the probability of an offspring having a particular genotype/phenotype.
The rows represent the possible gametes of one parent

The columns represent the possible gametes of another parent

The boxes represent the possible genotypes of offspring.

Question 16

Recessive genetic disorders:

Answer 16

To have a recessive disorder (express it in their phenotype) an individual must be homozygous recessive.
If an individual is heterozygous for the disease they are referred to as Carriers, but the disease is not expressed in their phenotype.

Question 17

Dominant Genetic Disorders

Answer 17

Disorder expressed if the individual has both dominant alleles or is heterozygous as it is caused by the dominant allele.

Question 18

Inbreeding increases the prevalence of recessive disorders

Answer 18

If a genetic disorder exists within a family the frequency of that disorder being expressed phenotypically dramatically increases if there is inbreeding within that family. More chances that the 2 parents will have at least one copy of the recessive allele.

Question 19

Pedigree?

Answer 19

A family tree showing the genetic connections amongst the family members.

Pedigrees are useful in following recessive genetic disorders as their may be several family members that do not express the disorder, but are carries (heterozygotes).