inheritance

gamete

the reproductive cells of an organism

haploid

contains half the chromosome number (occurs during meiosis)

diploid

- each cell has two sets of homologous chromosomes
- the alleles at a specific locus may be either homozygous or heterozygous

chromatin

DNA wound around histone proteins

chromosome

- long linear DNA wound around histones
- in the form of sister chromatids joined by a centromere

autosomal chromosome

a specific gene that is not on a sex chromosome and is a numbered chromosome

somatic chromosome

any chromosome that is not a sex chromosome

homologous chromosome

- a matching pair of chromosomes (one maternal, one paternal)
- same genes at the same position of each chromosome

gene locus

the position of a particular gene on the chromosome

nucleotide

monomer of DNA/RNA

gene

a section of DNA: sequence of nucleotide bases that code for a polypeptide or functional RNA

allele

different versions of one gene

multiple alleles

• where there are more than 2 allelic forms of a gene

• e.g. I^A, I^B, I^O

dominant

an allele that is always expressed in the phenotype even in heterozygous

recessive

the allele that is only expressed when homozygous (only expressed in the absence of the dominant version)

what is meant by codominant alleles?

• both alleles are expressed and contribute to the phenotype

• e.g. C^RC^W x C^RC^W

homozygous

when the two alleles the organism contains are the same

heterozygous

when the two alleles the organism contains are different

pure breeding

if two individuals who are both identically homozygous for a particular characteristic are bred together, they will produce offspring with exactly the same genotype and phenotype as the parents

genotype

the genetic constitution of an organism

phenotype

- the effect of the environment on the genotype
- expression due to genotype

monohybrid

• 1 gene, 2 alleles

• e.g. Aa x Aa

sex linkage

- 1 gene located on X or Y chromosome
- 2 alleles
- e.g. X^NXⁿ x X^NY^-

dihybrid

- 2 genes, 2 alleles each on separate chromosomes
- e.g. AaBb x AaBb
- 4 gametes
- expected phenotype ratios: 9:3:3:1

epistasis

• 2 genes, 2 alleles each

• e.g. AaBb x AaBb

• 4 gametes

• 1 gene masks the effect of the other

• don’t get the expected genotype: NOT 9:3:3:1

autosomal linkage

- 2 genes, 2 alleles each
- e.g. AaBb x AaBb
- both genes located on same chromosome
- don’t get the expected genotype, e.g. NOT 9:3:3:1
- 2 gametes (as the genes are linked on 1 chromosome)

in genetic crosses, why are the observed phenotypic ratios obtained in the offspring often not the same as the expected ratios?

1. small sample size
2. fertilisation of gametes is random
3. linked genes
4. epistasis
5. lethal genotypes

chi squared

1. compare the calculated test result to critical value
2. probability of chance for your test result - compare the probability from the table to 5%

sex linked gene

any gene located on the X or Y chromosome

a recessive allele which has harmful effects is able to reach a higher frequency in a population than a harmful dominant allele. explain how. (3)

1. recessive alleles can be carried by individuals without showing effects
2. organisms that are carriers more likely to reproduce
3. therefore, recessive alleles are more likely to be passed on

suggest why a sex linked disease is more common in males. (2)

1. males only have one X chromosome
2. a single copy of the recessive allele will be expressed

statistical test result < critical value

- accept null hypothesis
- there is no significant difference
- greater than 5% probability that the difference in results is due to chance

statistical test result > critical value

- reject null hypothesis
- there is a significant difference
- less than 5% probability that the difference in results is due to chance