Topic 10: Genetics HL

Meiosis

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

The chromosomes

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

Chiasmata is formed during prophase I of meiosis whereby homologous chromosomes pair up in a process called synapsis. The chromatids break and and non-sister chromatids join up. The X-shaped structure of chiasmata is then formed at the position where crossing over occurs. Chiasmata become visible when homologous chromosomes unpair. The chiasma holds homologous chromosomes together until anaphase

crossing over/chiasmata formed during prophase I of meiosis;
pairing of homologous chromosomes/synapsis;
chromatids break
non-sister chromatids join up/swap/exchange alleles/parts;
X-shaped structure formed / chiasmata are X-shaped structures;
chiasma formed at position where crossing over occurred;
chiasmata become visible when homologous chromosomes unpair;
chiasma holds homologous chromosomes together (until anaphase);
Accept the above points in an appropriately annotated diagram.

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. 

[Nov 2012] Explain how meiosis results in an effectively infinite genetic variety of gametes.

10.1.4 State Mendel’s law of independent assortment.
10.1.5 Explain the relationship between Mendel’s law of independent assortment
and meiosis.

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.



10.2.2 Distinguish between autosomes and sex chromosomes. [4 marks on Nov 2012 paper]

X and Y chromosomes determine sex. Females are XX and males are XY.
X chromosome is larger and carries more genes than the Y chromosome
Both males and females have same types of autosomes, of which both genders have 22 pairs.

X and Y chromosomes determine sex;
females XX and male XY;
X chromosomes is larger than / carries more genes than the Y chromosome;
22 types/pairs of autosomes;
males and females have same types of autosomes;

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.


10.2.4 Define linkage group.


10.2.5 Explain an example of a cross between two linked genes.
Alleles are usually shown side by side in dihybrid crosses, for
example, TtBb. In representing crosses involving linkage, it is
more common to show them as vertical pairs, for example
 
This format will be used in examination papers, or students will
be given sufficient information to allow them to deduce which
alleles are linked.
10.2.6 Identify which of the offspring are recombinants in a dihybrid cross
involving linked genes.
In a test cross of  
the recombinants will be  
and  

Polygenic inheritance

10.3.1 Define polygenic inheritance.

Polygenic inheritance - Two or more genes control

character affected/influenced/defined/determined/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.
Aim 8: This is one of the most obvious opportunities to develop
the theme of parity of esteem for all humans. The selective
advantage of dark skin to protect against ultraviolet light and
light skin to allow vitamin D production could be mentioned.
The correlation between skin colour and intensity of sunlight
is clear, though the selective advantages of particular skin
colours can now be overcome by the use of sun-block creams
and vitamin D supplements.

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. Obj 3
10.2.2 Distinguish between autosomes and sex chromosomes. Obj 2
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. Obj 3
10.2.4 Define linkage group. Obj 1
10.2.5 Explain an example of a cross between two linked genes. Obj 3
Alleles are usually shown side by side in dihybrid crosses, for
example, TtBb. In representing crosses involving linkage, it is
more common to show them as vertical pairs, for example

This format will be used in examination papers, or students will
be given sufficient information to allow them to deduce which
alleles are linked.
10.2.6 Identify which of the offspring are recombinants in a dihybrid cross
involving linked genes.