Genetics revision 1.

Target Language or Knowledge:

Huntington’s disease is a genetic disorder in humans. It is caused by a dominant affected allele (H). The normal allele is recessive (h). (a) State the possible genotypes and phenotypes for Huntington’s disease.
_hh_ - unaffected by Huntington's / normal _ Hh_ - affected by Huntington's HH - _affected_ by Huntington's
Explain how the two alleles, H and h, combine to produce different phenotypes. In your answer you should: • define phenotype and genotype • explain how the three genotypes code for only two phenotypes.
Genotype: the combination of two _alleles_ coding for a _trait._ Phenotype: the _physical_ representation of the genotype. In Hh the H allele is _dominant_ over the h allele and masks it; thus the _phenotype_ of this individual is affected by Huntington's. hh genotype codes for the phenotype to be _unaffected_ by Huntington's. Both recessive alleles _hh_ must be present to produce the _phenotype_ unaffected by Huntington's. HH and Hh genotypes code for the phenotype to be _affected_ by Huntington's.
Huntington’s disease occurs when a mutation causes a dominant affected allele (H). The normal allele is recessive (h). Using Huntington’s disease as an example and the diagram opposite, explain the relationship between DNA, genes, alleles, mutations, and phenotype.
DNA - contains the _genetic_ _instructions._ Gene - a section of _DNA_ that codes for a particular _protein_ or characteristic. Alleles are different versions of a _gene_ that have a different _base_ sequence. Alleles in this case will code for the _phenotypes_ affected and unaffected by Huntington's. A mutation is a _change_ in the order of DNA _bases,_ which causes a _new_ allele. The new _allele_ codes for a different protein resulting in the phenotype of Huntington's.
Polycystic kidney disease is caused by a dominant allele (D) and the unaffected allele is recessive (d).
List the genotypes of the following two individuals:. 3 : _Dd_ 4 : _dd_ Explain how you worked out the genotype for individual 3. You should support your answer using evidence from BOTH the parents AND children of individual 3. 3 has the genotype _Dd._ Parent 2 could either pass down a _D_ or _d_ allele because it has the genotype _Dd_ heterozygous. Parent 2 must have passed down a _D_ allele for individual 3 to be affected by Polycystic kidney disease. Parent 1 must have passed down a _d_ recessive allele to 3 because it has the genotype _d_ homozygous _recessive_ and only has _d_ recessive _alleles_ to pass on. The offspring of cross 3 and 4 are both affected and unaffected; this proves that 3 has the genotype _Dd_ and not _DD._ Offspring 7 is unaffected dd, and thus must have inherited a _d_ allele from both parents 3 and 4. This proves 3 must have a _d_ allele to pass onto offspring 7 and it must have a _D_ allele because it is affected. Thus it proves 3 has the genotype _Dd_
One of the family members has kidney failure as a result of an infection. Discuss if this type of kidney failure can be inherited by any future children they have.
The kidney failure was due to an infection. Kidney failure was caused by _'environment',_ not _genetics._ Only _genetic_ characteristics can be _inherited,_ not those acquired as a result of _environment._ Reproductive cells (gametes) can only pass on _genetic_ characteristics and thus kidney failure caused by an infection will _not_ pass to _offspring_
Explain how sexual reproduction causes genetic variation in a hoiho ( penguin) population. In your answer you should consider: • the processes of gamete formation (meiosis) and fertilisation
Gamete formation by meiosis. produces _gametes_ / sex cells which have _half_ the normal number of _chromosomes_ as body cells. Description of meiosis: 1 cell producing 4 _gametes._ _ Random_ assortment and _crossing-over_ of chromosomes producing genetically _unique_ gametes. Process of fertilisation: Random _male_ and female _gametes_ join, each with _unique_ DNA, producing a genetically unique _offspring._ Role of sexual reproduction: Random fertilisation produce new combination of _alleles_ and thus genetic variation between _individuals_
Discuss how genetic variation could lead to increased survival of the hoiho population when faced with avian malaria.
The advantage of genetic variation to a species is that it may enable some individuals to _survive_ the parasitic infection and reproduce, passing on _favourable_ alleles or _genes_ to the next generation. Over many _generations_ this genetic advantage / genes / _alleles_ will rise in the _population,_ allowing _survival_ of the hoiho species.