Chromosomal Inheritance Overview

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Overview of chromosomal inheritance.

The chromosome theory of inheritance states that the Mendelian genes are present at specific locations (loci) on chromosomes

It is the chromosomes themselves that are subject to segregation and independent assortment.

Wild-type genes and traits are those that are most commonly found within a natural population.

Steps of chromosomal inheritance via Independent Assortment:

• The genotypes of two parental organisms are big D, big D, big R, big R and little d, little d, little r, little r.
• The genotypes of the F1 generation gametes are Big D, big R; big D, little r; little d, big R; little d, little r.
  • Thus, we see that each genotype has an equal chance (1 in 4) of occurring.

Exceptions:

Linked Genes

• Because a single chromosomes contain many genes, depending on the traits studied, some genes may be linked by being on the same chromosome.
• Linked genes are located on the same chromosome and so, are inherited together.

Crossing Over

• Crossing over is a process in which the physical connection of genes on the same chromosome is broken.
• These are the results of meiosis and normal segregation of chromosomes.
• But sometimes during meiosis 1, bits of two chromatids are broken off and are rejoined to the other chromatid.

Sex-linked genes are those located on either sex chromosome.

• These genes exhibit special inheritance patterns.

Full-Length Text

• Here we will learn an overview of chromosomal inheritance.

• To begin, let's start a table to understand some key concepts.

• Denote that the chromosome theory of inheritance states that the Mendelian genes are present at specific locations (loci) on chromosomes, and that it is the chromosomes themselves that are subject to segregation and independent assortment.

• Denote that wild-type genes and traits are those that are most commonly found within a natural population;

Let's illustrate the steps of chromosomal inheritance.

• Write that the genotypes of two parental organisms are big D, big D, big R, big R and little d, little d, little r, little r.

• Draw two circles to represent the gametes each organism produces.

• Draw two different chromosomes in each gamete.

• Label a single gene on each chromosome (big D, then big R in one gamete and little d, then little r in the other gamete).

• Draw a circle to represent the F1 generation.

• Within the circle, draw four chromosomes and label the four genes present.

• Now draw four circles to represent the potential gametes produced by the F1 generation.

• Within each circle, draw two chromosomes.

• Label the genes on the chromosomes: big D, big R; big D, little r; little d, big R; little d, little r.
  • Thus, we see that each genotype has an equal chance (1 in 4) of occurring.

We previously learned Mendel's concept of Independent Assortment. However, it turns out that the concept doesn't quite mirror biology. Because a single chromosomes contain many genes, depending on the traits studied, some genes may be linked by being on the same chromosome.

• Denote that linked genes are located on the same chromosome and so, are inherited together.

Let's look at this concept of linked genes more closely.

• Draw two circles to represent the parental generation (the first is male the second female).

• Within each circle, draw two homologous chromosomes.
  • Use different colors to signify which chromosomes are maternal and which are paternal.

• In the first circle, label two genes on each chromosome: big D, big A and little d, little a.

• In the second circle, also label two genes on each chromosome: little d, little a and little d, little a.

• Draw two circles to represent the possible offspring.

• Within the circles draw two chromosomes, one from each parent.

• Label the genes on each chromosome: big D, big A, little d, little a, and little d, little a, little d, little a.

• Because big D and big A are on the same chromosome, they segregate together which is why there isn't a chromosome that is big D, little a or little d, big A.
  • Except if crossing over occurs.

• Denote that crossing over is a process in which the physical connection of genes on the same chromosome is broken.

Let's explore crossing over.

• Draw a circle to represent a diploid organism.

• Draw two homologous chromosomes within this organism.
  • Color them differently which will come in handy later.

• Label the genes on each chromosome: first big D, big A and then little d, little a.

• Draw two circle to represent two of the possible gametes produced by these linked genes.

• Draw and label the single chromosomes in each gamete: big D, big A and little d, little A.
  • These are the results of meiosis and normal segregation of chromosomes.
  • But sometimes during meiosis 1, bits of two chromatids are broken off and are rejoined to the other chromatid.

• Draw two more gamete circles.

• Within each circle draw a chromosome.
  • The color at the bottom of each chromosome is switched due to the crossing over event.

• Label the chromosome in one big D, little a.

• Label the chromosome in the other little d, big A.
  • We will discuss this process more in depth elsewhere.

• Finally, there is a special form of chromosomal inheritance related to the genes located on the sex chromosomes (X chromosome and Y chromosome).

• Denote that sex-linked genes are those located on either sex chromosome.

• Denote that these genes exhibit special inheritance patterns.

We will delve into these patterns elsewhere.

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