Biology IB HL - 3.4 Inheritance Part 2

A monohybrid cross determines the allele combinations for potential offspring for one gene only

Step 1: Designate letters to represent alleles (dominant = capital letter ; recessive = lower case ; co-dominant = superscript)

Step 2: Write down the genotype and phenotype of the prospective parents (this is the P generation)

Step 3: Write down the genotype of the parental gametes (these will be haploid and thus consist of a single allele each)

Step 4: Draw a grid with maternal gametes along the top and paternal gametes along the left (this is a Punnett grid)

Step 5: Complete the Punnett grid to determine potential genotypes and phenotypes of offspring (this is the F1 generation)

The genotypic and phenotypic ratios calculated via Punnett grids are only probabilities and may not always reflect actual trends

E.g. When flipping a coin there is a 50% chance of landing on heads – this doesn’t mean you will land on heads 50% of the time

Mendel crossed different varieties of pea plants and recorded the characteristics of resultant offspring

Initially, he crossed purebred dominant and purebred recessive plants in order to produce heterozygotes (F1 generation)

He then self-pollinated the heterozygotes to produce an F2 generation and counted the dominant and recessive phenotypes

The expected ratio of dominant : recessive phenotypes was 3 : 1 – this ratio was supported by the experimental data

more data, more likely to get closer to this ratio

Genetic diseases are caused when mutations to a gene (or genes) abrogate normal cellular function, leading to the development of a disease phenotype

Genetic diseases can be caused by recessive, dominant or co-dominant alleles

An autosomal recessive genetic disease will only occur if both alleles are faulty

Heterozygous individuals will possess one copy of the faulty allele but not develop disease symptoms (they are carriers)

An example of an autosomal recessive genetic disease is cystic fibrosis

An autosomal dominant genetic disease only requires one copy of a faulty allele to cause the disorder

Homozygous dominant and heterozygous individuals will both develop the full range of disease symptoms

An example of an autosomal dominant genetic disease is Huntington’s disease

If a genetic disease is caused by co-dominant alleles it will also only require one copy of the faulty allele to occur

However, heterozygous individuals will have milder symptoms due to the moderating influence of a normal allele

An example of a genetic disease that displays co-dominance is sickle cell anaemia

Cystic fibrosis is an autosomal recessive disorder caused by a mutation to the CFTR gene on chromosome 7

Individuals with cystic fibrosis produce mucus which is unusually thick and sticky

This mucus clogs the airways and secretory ducts of the digestive system, leading to respiratory failure and pancreatic cysts

NO

Huntington’s disease is an autosomal dominant disorder caused by a mutation to the Huntingtin (HTT) gene on chromosome 4

The HTT gene possesses a repeating trinucleotide sequence (CAG) that is usually present in low amounts (10 – 25 repeats)

More than 28 CAG repeats is unstable and causes the sequence to amplify (produce even more repeats)

When the number of repeats exceeds ~40, the huntingtin protein will misfold and cause neurodegeneration

This usually occurs in late adulthood and so symptoms usually develop noticeably in a person’s middle age (~40 years)

Symptoms of Huntington’s disease include uncontrollable, spasmodic movements (chorea) and dementia

There are over 4,000 identified single gene defects that lead to genetic disease, but most are very rare

Any allele that adversely affects survival and hence the capacity to reproduce is unlikely to be passed on to offspring

Recessive conditions tend to be more common, as the faulty allele can be present in carriers without causing disease

Dominant conditions may often have a late onset, as this does not prevent reproduction and the transfer of the faulty allele

Sex linkage refers to when a gene controlling a characteristic is located on a sex chromosome (X or Y)

The Y chromosome is much shorter than the X chromosome and contains only a few genes (50 million bp; 78 genes)

The X chromosome is longer and contains many genes not present on the Y chromosomes (153 million bp ; ~ 2,000 genes)

Hence, sex-linked conditions are usually X-linked - as very few genes exist on the shorter Y chromosome

Sex-linked inheritance patterns differ from autosomal patterns due to the fact that the chromosomes aren’t paired in males (XY)

This leads to the expression of sex-linked traits being predominantly associated with a particularly gender

As human females have two X chromosomes (and therefore two alleles), they can be either homozygous or heterozygous

Hence, X-linked dominant traits are more common in females (as either allele may be dominant and cause disease)

Human males have only one X chromosome (and therefore only one allele) and are hemizygous for X-linked traits

X-linked recessive traits are more common in males, as the condition cannot be masked by a second allele

Only females can be carriers (a heterozygote for a recessive disease condition), males cannot be heterozygous carriers

Males will always inherit an X-linked trait from their mother (they inherit a Y chromosome from their father)

Females cannot inherit an X-linked recessive condition from an unaffected father (must receive his dominant allele)

Red-green colour blindness and haemophilia are both examples of X-linked recessive conditions

Consequently, they are both far more common in males than in females (males cannot mask the trait as a carrier)

When assigning alleles for a sex-linked trait, the convention is to write the allele as a superscript to the sex chromosome (X)

Haemophilia: XH = unaffected (normal blood clotting) ; Xh = affected (haemophilia)

Colour blindness: XA = unaffected (normal vision) ; Xa = affected (colour blindness)

Haemophilia is a genetic disorder whereby the body’s ability to control blood clotting (and hence stop bleeding) is impaired