Biology - 13 Reproduction - 13.7 Inheritance in Action & 13.8 More About Genetics & 13.9 Inherited Disorders & 13.10 Screening for Genetic Disorders

Allele

alternative form of a gene coding for the same characteristic

Recessive

only present in the phenotype if present on both chromosomes

Dominant

present in the phenotype even when only present on one chromosome

Genotype

an organism's allele combinations or genetic makeup

Phenotype

physical characteristics of an organism

Homozygous

having two identical alleles for a particular gene

Heterozygous

having two different alleles for a particular gene

Punnett Square

diagram showing the gene combinations that might result from a genetic cross

Genetic cross

considering the potential offspring that may result from two known parents

Genetic cross diagram

genetic diagram showing how alleles are passed down

Polydactyly

genetic condition causing extra digits

Polydactyly is generally caused by a ... allele

dominant

Cystic fibrosis

genetic disorder of the cell membranes causing thick, sticky mucus affecting exchange over surfaces

What parts of the body does cystic fibrosis affect? [3]

- lungs
- digestive system
- reproductive organs

Cystic fibrosis: lungs

excess mucus obstructs air flow

Cystic fibrosis: digestive system [2]

- mucus blocks enzymes from pancreas reaching small intestine
- food is not fully digested

Cystic fibrosis: reproductive organs [2]

- mucus can obstruct sperm duct or oviduct
- affects fertility

Cystic fibrosis is generally caused by a ... allele

recessive

Carrier

person whose genotype includes a recessive allele not expressed in the phenotype

Screening

to test or examine for the presence of a disorder

Amniocentesis

a needle is used to collect foetal cells from the placenta and foetus in the amniotic fluid

Chorionic Villus Sampling (CVS)

a catheter is used to take a sample of placental tissue through the vagina or wall of the abdomen

Pre-Implantation Genetic Diagnosis (PIGD)

embryos from IVF are screened and healthy ones are chosen to be implanted

When is amniocentesis carried out?

15-16 weeks

When is CVS carried out?

10-12 weeks

Risk of miscarriage with amniocentesis:

0.5 - 1%

Risk of miscarriage with CVS:

1 - 2 %

Advantages of genetic screening [3]:

- accurate and reliable
- can prevent suffering
- caring for genetic disorders is costly and emotionally taxing

Disadvantages of genetic screening [5]:

- increase risk of miscarriage
- occasional false positives and negatives
- couples must decide whether to terminate a pregnancy
- screening is costly
- could be used to create only 'desirable' offspring