Genetic Continuity: Mendelian's Theory

Study Notes

Mendelian's Theory

A monohybrid cross involves the cross between one trait, and it follows the Law of Segregation, where allele pairs separate and go into separate gametes.
A dihybrid cross involves the cross between two traits, and it follows the Law of Independent Assortment (2nd Law), where alleles of different genes for separate traits are passed independently of one another from parents to offspring during gamete formation.
The Law of Independent Assortment occurs in metaphase I of meiosis and results in a 9:3:3:1 ratio.

Gene Recombination/Crossing Over

Gene recombination, or crossing over, is the production of offspring with combinations of traits that differ from those found in either parent.
Crossing over occurs at prophase 1 of meiosis 1, resulting in a mixture of parental characteristics in offspring.
The point of crossing over is called a chiasma, and genes that are far away from each other have the highest chances of crossing over.

Sex-Linked Genes

Sex-linked genes are genes that are carried by either X or Y sex chromosomes.
Sex-linked conditions include color-blindness, haemophilia, and baldness, and are more common in males because they have only one X chromosome.

Variability

Sexual reproduction increases genetic diversity through the random combination of maternal and paternal chromosomes during meiosis.
Random fertilization of the male and female gamete ensures each gamete has a unique set of genes.
Gene recombination, or crossing over, increases genetic diversity by creating new combinations of genes during meiosis.
The Laws of Segregation and Independent Assortment describe the separation and combination of alleles during gametogenesis.
Mutation leads to variation by producing random changes in an organism's genes/DNA bases/genome, causing different proteins to be formed or new alleles to be formed.

Types of Variation

There are two types of variation: continuous and discontinuous.
Continuous variation is when individuals cannot be placed in distinct categories, e.g., height.
Discontinuous variation is when individuals can be placed into distinct categories, e.g., blood group.

Determination of Sex

Chromosomal determination of sex involves the 23rd pair of chromosomes.
In females, a barrbody (tiny dark object) is formed by condensation of X chromosomes during interphase.

Genetic Disorders

Genetic disorders are diseases caused by an abnormality in an individual's DNA.
There are two types of chromosomes associated with disorders: autosomal and sex chromosomes.
Autosomal disorders include Down's syndrome (trisomy 21), and sex chromosomal disorders include Turner's syndrome (45, X0) and Klinefelter's syndrome (XXY).
Monosomy is the most serious because it loses the chromosomes that affect genes.
Trisomy is when a person has three copies of chromosomes.
Non-disjunction is the failure of chromosomes to separate.
Autosomal non-disjunction involves 1-22 pair of chromosomes and can result in disorders such as Down's syndrome (trisomy 21).

Description

This quiz covers Mendelian's Theory of Genetic Continuity, including monohybrid and dihybrid cross, and the laws of segregation and independent assortment. Understand the principles of inheritance and gamete formation.