IB Categories Archives: Topic 10: Genetics and evolution

10.3 – Polygenic Inheritance

10.3 – Polygenic Inheritance

10.3.1 – Define polygenic inheritance

Inheritance of phenotypic characters (such as height, eye colour in humans) that are determined by the collective effects of several genes. A single characteristic that is controlled by two or more genes.

10.3.2 – Explain that polygenic inheritance can contribute to continuous variation using two examples, one of which must be human skin colour

Since a single characteristic may be influenced by more than one gene, it may exhibit continuous variation within a population. These genes are collectively called polygenes. Each allele of a polygenic character often contributes only a small amount to the overall phenotype, making study of individual alleles difficult. Phenotypic variation is the result of genotypic variation coupled with environmental variation. Environmental effects smooth out the genotypic variation, giving continuous distribution curves.

Skin Colour

Skin colour is actually determined by more than two genes. However, this example shows only two. It is represented by nine possible genotypes, which form five phenotypes. There are two genes, each with two alleles that control the amount of melanin

There are two genes, each with two alleles that control the amount of melanin A and B code to add melanin

A and B code to add melanin a and b code for no added melanin

a and b code for no added melanin



The amount of pigment produced is directly proportional to the number of dominant alleles for either gene. Having no dominant alleles results in an albino.




However, the phenotype is also influenced by environmental factors. In the case of skin colour, the exposure of the individual to sunlight will slightly alter the amount of melanin produced in their skin. This smooths out the distribution of skin colour into one continuous curve.


Finch Beak Depth

Finches eat seeds, breaking them open with their beaks. The depth of a finch’s beak is controlled by a number of genes; here, we only look at two: A and B code to add

A and B code to add depth
a and b code for no added depth.

Heterozygous cross: AaBb x AaBb

Darker orange indicates greater beak depth. This will form a similar distribution to that of skin colour seen above. Once again, environmental factors will smooth out the distribution.


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10.2 – Dihybrid Crosses and Gene Linkage

10.2 – Dihybrid Crosses and Gene Linkage

10.2.1 – Calculate and predict the genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes

A dihybrid cross is a cross involving two genes that control two different characteristics. Unlinked genes are found on different chromosome, so they will be separated by random assortment during meiosis. Autosomal genes are found on any chromosomes other than the XY gender-determining chromosomes.

According to the law of independent assortment, each pair of alleles segregates into gametes independently. This means that different types of gametes can form. Therefore, a cross between two heterozygous parents always has the ratio 9 : 3 : 3 : 1 

For example F1 Cross

F1 Cross

Parents – Father: Homozygous black, short hair: BBLL
Mother: Homozygous red, long hair: bbll

Possible Gametes

The only possible genotypes for the offspring are: BbLl.
The offspring are heterozygous for both traits, expressing only the dominant allele.

F2 Cross

Parents – Father: Heterozygous black, short hair: BbLl
Mother: Heterozygous black, short hair: BbLl
 Possible Gametes –

The dihybrid cross would look like:

9 : 3 : 3 : 1 is the F2 dihybrid ratio. It is a prediction of the offspring ratio, but may differ in real life.

Test Cross

To test if an organism is heterozygous for particular traits, a test cross is done. The organism is crossed with a homozygous recessive. If it is heterozygous, the ratio will be 1 : 1 : 1 : 1

This is called the back-cross ratio or dihybrid test ratio.

10.2.2 – Distinguish between autosomes and sex chromosomes

Autosome – A chromosome that is not a sex-chromosome. They do not vary depending on gender.

Sex Chromosome – A chromosome which determines sex rather than other body (soma) characteristics.

10.2.3 – Explain how crossing over between non-sister chromatids of a homologous pair in prophase I can result in an exchange of alleles

During prophase I, the homologous pairs of chromosomes pair up and are in close proximity to each other. Breakages may occur along the chromatids, allowing fragments to be exchanged between the non-sister chromatids. The rejoining of non-sister chromatids forms chiasmata, which remain intact until the chromosomes are separated by the microtubules.

As a result of crossing over, recombination of linked genes can occur.

10.2.4 – Define linkage group

Linkage group – the genes carried on any one chromosome

These tend to be inherited together, which results in fewer genetic combinations

10.2.5 – Explain an example of a cross between two linked genes

Sweet Peas: Lathyrus odoratus

The F2 cross is done between the heterozygous parents. In this case, the gametes formed during meiosis can be groups into two categories:

The majority of the offspring in the F2 cross will have genotypes that match the parents, such as:

The third genotype seen above is the result of random assortment of the chromosomes during meiosis.


However, there may also be a low frequency of recombinant genotypes as a result of crossing over:


10.2.6 – Identify which of the offspring are recombinants in a dihybrid cross involving linked genes

Recombinants can be recognised by unpredicted combinations of characteristics, low frequency of new combinations of phenotype and statistically significant difference from the ratio expected.

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10.1 – Meiosis

10.1 – Meiosis

10.1.1 – Describe the behaviour of the chromosomes in the phases of meiosis

Meiosis consists of two nuclear divisions but only one replication of the chromosomes

Meiosis I

Prophase I

The chromosomes condense and become visible and centrioles duplicate. The homologous will chromosomes pair up, forming bivalents. Chiasmata, where crossing over has occurred, become visible. The centrioles move to the poles of the cell and the nuclear membrane begins to break down.




Metaphase I

The chromosomes become shorter and thicker and the spindle microtubules attach to the centromeres. The bivalents lines up along the equator. Those with chiasmata temporarily have unusual shapes. The spindles attach to the chromosomes and start to pull them apart.


Anaphase I

The spindle fibres begin to shorten, separating the homologous chromosomes. They are pulled to opposites poles, thereby halving the chromosome number. Each of the chromosomes is made up of two chromatids; however, these might not be identical due to crossing over.


Telophase I

The nuclear membranes begin to reform around the chromosomes. The cell membrane closes, dividing into two cells. The chromosomes begin to uncoil partially. The new cells may enter a short interphase, but there will be no further replication of the DNA.


Meiosis II

Prophase II

The chromosomes recoil, becoming short and thick once again. The centrioles replicate and move to opposite poles. The nuclear membranes will break down.


Metaphase II

The spindle fibres begin to form and attach to the chromosomes by the centromere. The chromosomes line up along the equator. The centromeres divide to allow the chromosomes to separate.

Anaphase II

The sister chromatids are separated, pulled apart by the spindle fibres to the opposite poles.





Telophase II

The chromatids decondense and are now known as chromosomes. The nuclear membrane and nucleoli reform around the DNA. The cells divide, forming a total of four haploid cells.



10.1.2 – Outline formation of chiasmata in the process of crossing over

Breakages of the chromatids occur frequently during the coiling and shortening process, but this is immediately repaired. However, since the homologous chromosomes are paired closely together, these fragments may swap between non-sister chromatids. The place where the crossing over occurs is called the chiasma (plural: chiasmata). This results in new combinations of genes.



10.1.3 – Explain how meiosis results in an effectively infinite genetic variety in gametes through crossing over in prophase I and random orientation in metaphase I


This is when fragments of DNA are swapped between homologous chromosomes, forming new combinations of linked genes. The new genotypes are not due to random assortment. There are virtually unlimited possibilities of recombinations.

Random Orientation

During anaphase I, the bivalents line up randomly along the equator. The homologous pairs and their alleles are separated. However, the chromosomes move independently of each other, so there is no relationship between the chromosome and which pole it moves to. A total of 223 combinations can be formed in humans.

10.1.4 – State Mendel’s law of independent assortment

Allele pairs separate independently during the formation of gametes. If two genes are unlinked, then the pairs of alleles will be segregated randomly during meiosis. As a result, traits are transmitted to offspring independently of one another

10.1.5 – Explain the relationship between Mendel’s law of independent assortment and meiosis

Independent assortment is the result of random orientation of the homologous chromosomes during metaphase I. Mendel’s discoveries were based on physical characteristics, or phenotypes, and not the actual alleles or chromosomes.


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