Topic 10.3: Polygenic inheritance

10.3.1 Define polygenic inheritance

Polygenic inheritance refers to a single characteristic that is controlled by more than two genes (also called multifactorial inheritance)

Polygenic inheritance patterns normally (bell-shaped) distribution curve - it shows continuous variation

By increasing the number of genes controlling a trait, the number of phenotype combinations also increase, until the number of phenotypes to which an individual can be assigned are no longer discrete, but continuous

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

Human skin colour

• The colour of human skin is determined by the amount of dark pigment (melanin) it contains
• At least four (possibly more) genes are involved in melanin production; for each gene one allele codes for melanin production, the other does not
• The combination of melanin producing alleles determines the degree of pigmentation, leading to continuous variation

Grain colour in wheat

• Wheat grains vary in colour from white to dark red, depending on the amount of red pigment they contain
• Three genes control the colour and each gene has two alleles (one coding for red pigment, the other coding for no pigment)
• The most frequent combinations have an equal number of "pigment producing" and "no pigment" alleles, whereas combinations of one extreme or the other are relatively rare
• The overall pattern of inheritance shows continuous variation

Topic 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 determines the allele combinations of offspring for two particular genes that are unlinked (not on the same chromosomes)

Because there are two genes with two alleles per gene (multiple alleles not required), there can be up to four different gamete combinations

10.2.2 Distinguish between autosomes and sex chromosomes

Autosomes: Pairs of chromosomes that are identical in appearance (e.g. same size, same gene loci, etc) and are not involved in sex determination

Sex chromosome: Pairs of chromosomes involved in sex determination and are not identical in appearance (e.g. X and Y chromosome in humans)

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

During crossing over in prophase I, non-sister chromatids of a homologous pair may break and reform at points of attachment called chiasmata

As these chromatids break at the same point, any gene loci below the point of the break will be exchanged as a result of recombination

This means that maternal and paternal alleles may be exchanged between the maternal and paternal chromosomes, creating new gene combinations

The further apart two gene loci are on a chromosome, the more likely they are to be exchanged

10.2.4 Define linkage group

A linkage group is a group of genes whose loci are on the same chromosome and therefore do not follow the law of independent assortment

Linkage genes will tend to be inherited together - the only way to separate them is through recombination (via crossing over during synapsis)

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

When two genes are linked, they do not follow the expected phenotypic ratio for a dihybrid cross between heterozygous parents

Instead the phenotypic ratio will follow that of a monohybrid cross as the two genes are inherited together

This means that offspring will tend to produce the parental phenotypes

Recombinant phenotypes will only be evident if crossing over occurs in prophase I and would thus be expected to appear in low number (if at all)

An example of a cross between two linked genes is the mating of a grey bodied, normal wing fruit fly with a black bodied, vestigial wing mutant

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

Recombinants of linked genes are those combinations of genes not found in parents

For example, in a test cross of a heterozygous fruit fly (grey bodied, normal wings) with a homozygous recessive mutant (black bodied, vestigial wings), the recombinants would be the grey bodied, vestigial winged offsprings and the black bodied, normal winged offspring

Linked genes that have undergone recombination can be distinguished from unlinked genes via test cross because the frequency of the recombinant genotype will always be less than would occur for unlinked genes (crossing over does not happen every time)

• For example:
• Heterozygous test cross of unlinked genes = 1:1:1:1 phenotypic ratio
• Heterozygous test cross of linked genes = 1:1:0.1:0.1 pehnotypic ratio (uncommon phenotypes are recombinants)

Topic 10.1: Meiosis

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

Interphase: Cell growth and DNA replication (duplication of DNA creates sister chromatid chromosome)


Meiosis I

• Prophase I: DNA supercoils and chromosomes condense, nuclear membrane dissolves, homologous pairs form bivalents, crossing over occurs
• Metaphase I: Spindle fibres from centrioles (at poles) attach to centromeres of bivalent, bivalents line up along the equator of the cell
• Anaphase I: Spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell
• Telophase I: Chromosomes decondense, nuclear membranes may reform, cell divide (cytokinesis) forming two haploid daughters cells

Interkinesis: An optional rest period between meiosis I and meiosis II, no DNA replication occurs in this stage

Meiosis II

• Prophase II: Chromosomes condense, nuclear membrane dissolves (if reformed), centrioles move to opposite poles (perpendicular to previous poles)
• Metaphase II: Spindle fibres form centrioles attach to centromeres of chromosomes, chromosomes line up along the equator of the cell
• Anaphase II: Spindle fibres contract and split the chromosome into sister chromatids, chromatids (now called chromosomes) move to opposite poles
• Telophase II: Chromosomes decondense, nuclear membrane reforms, cells divide (cytokinesis), resulting in four haploid daughter cells

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

Crossing over involves the exchange of segments of DNA between homologous chromosomes during Prophase I of meiosis

The process of crossing over occurs as follows

• Homologous chromosomes become connected in a process called synapsis, forming a bivalent (or tetrad)
• Non-sister chromatids break and recombine with their homologous partner, effectively exchanging genetic material (crossing over)
• The non-sister chromatids remain connected in an X-shaped structure and the positions of attachment are called chiasmata

Chiasmata hold homologous chromosomes together as a bivalent until anaphase I

As a result of crossing over, chromatids may consist of a combination of DNA derived from both homologous - these are called recombinant

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

During anaphase I, homologous chromosomes separate, such that each resultant daughter (and subsequent gametes) contains a chromosome of either maternal or paternal origin

The orientation of these homologous in metaphase I is random, such that there is an equal probability of the daughter cell having either the maternal or paternal chromosome

As humans have a haploid number of 23 chromosomes, this means that there is 2²³ potential gamete combinations (over 8 million combinations)

Crossing over in prophase I results in entirely new chromosome combinations, as recombination through gene exchange produces wholly original chromosomes containing both maternal and paternal DNA, resulting in near infinite genetic variability

Other sources of genetic variation include random fertilisations, DNA mutations, chromosome mutations and non-disjunction

10.1.4 State Mendal's law of independent assortment

Gregor Mendal was a 19th century Moravian monk who demonstrated that the inheritance of traits (i.e. genes) followed particular laws:

• Law of segregation: Each herediatary characteristics is controlled by two alleles, which segregate and pass into different reproductive cells (gametes)
• Law of independent assortment: The separation of alleles for one gene will occur independently of the separation of alleles for another gene
• According to the law of independent assortment, different allele combinations should always be equally impossible
• However this law only holds for genes that are on different chromosomes - the law of independent assortment does not apply to linked genes

10.1.5 Explain the relationship between Mendal's law of independent assortment and meiosis

The law of independent assortment relates to the random orientation of homologous chromosomes in metaphase I of meiosis

Because the orientation of a homologous pair is random, and does not affect the orientation of any other homologous pair, any one of a pair of alleles on a chromosome has an equal chance of being paired with, or separated from, any one of a pair of alleles on another chromosome

This means the inheritance of two different traits will occur independently of each other (provided the genes aren't linked)

Topic 10: Genetics

Topic 10 of the IB HL Biology syllabus is the Genetics. IBO recommends to spend 6 hours on this topic.

This topic has 3 sub-chapters: "Meiosis", "Dihybrid crosses and gene linkage" and "Polygenic inheritance". Each are separated with numerical values in order of mentioned.

These are all HL syllabus statements, it is recommended to bring a Casio Graphical Calculator instead of Texas.