7.1 Inheritance Flashcards
What does homozygous mean?
having identical pairs of genes for any given pair of hereditary characteristics
What does heterozygous mean?
two different alleles for a hereditary characteristic
What is a phenotype?
visible characteristics.
the expression of an organisms genetic constitution, combined with its interaction with the environment
What is a genotype?
genetic makeup of a person
What is an allele?
different versions of a single gene found at the same locus on a chromosome. A single gene could have many alleles
Who is the father of modern genetics?
Gregor Mendel
What is cystic fibrosis?
a disease that causes thick and sticky mucus to coat the lungs, gut and pancreas. Its caused by recessive alleles FfxFf
What is Huntingdon’s disease?
a disease of the nervous system that causes shaking and eventually dementia. Its caused by a dominant allele Cc x cc
What is sickle cell anaemia?
a disease that alters the shape of red blood cells, thereby reducing their oxygen capacity, causing weakness and anaemia. Its caused by recessive alleles
What are genes and alleles?
- genes control the characteristics of an organism
- often genes have 2 different forms called alleles. For example if a gene controls the colour of a flower, the allele will control the exact colour
- In a pair, one allele may be dominant and another recessive
- Some genetic disorders are caused by recessive genes, others by dominant genes
What is a monohybrid cross?
- how a single character is passed on from one generation to another
- allow the genotype of offspring to be predicted.
What is a dihybrid cross?
where we consider the inheritance of 2 characteristics at the same time
What ratio does a monohybrid cross between 2 heterozygous produce?
3:1
What is independent assortment?
- The law of independent assortment means that genes separate independently of each other when gametes are made.
- it means that every possible combination of alleles for every gene is equally likely to happen. This means genes do not influence each other with regard to the sorting of alleles into gametes.
What is codominance?
- Codominance is where both alleles for the same characteristic are simultaneously expressed in the heterozygote.
- Neither of the alleles are recessive.
- it influences the phenotypic ratios of monohybrid and dihybrid crosses.
What does codominance change the phenotypic ration to?
1:2:1
What is autosomal linkage?
- Where two or more genes are located on the same chromosome that are close to one another are likely to be inherited together.
- Linkage shows that some allele combinations are not inherited independently of each other.
- Only 1 homologous pair is needed for all 4 alleles to be present. For genes that aren’t linked, 2 homologous pairs are needed
What is recombination?
- When gametes are produced by meiosis, multiple sections of homologous chromosomes are exchanged in a process called recombination.
- If two genes are located in close proximity on the same chromosome, they are more likely to be exchanged together and not separated in recombination.
- The genes are more likely to be transmitted to a gamete together.
What are autosomes?
- Autosomes are all the chromosomes except the sex chromosomes (X and Y).
- They are arranged in pairs called homologous chromosomes (one from the father and one from the mother).
- Homologous chromosomes consist of the same genes in the same order along the chromosome.
- There is some variation if the chromosomes have different alleles but the genes are the same.
What are sex chromosomes?
In many organisms, the sex chromosomes (X and Y) determine the sex of an individual.
Sex chromosomes differ from autosomes because they are non-homologous.
Non-homologous chromosomes do not consist of the same genes in the same order along the chromosome.
What is Hemizygosity?
Hemizygosity means that there is no role of dominance and recessiveness.
What are the features of haemophilia?
- X-linked blood disorder
- males are affected if they inherit one copy of the recessive allele
- females can be carriers if they are heterozygous
- females are only affected if they inherit 2 copies of the recessive allele
What are homologous chromosomes?
- Chromosomes in diploid organisms exist in homologous pairs.
- Homologous chromosomes consist of the same genes in the same order.
- The only variation between homologous chromosomes is in the alleles.
- Two alleles can be present for one gene.
What are multiple alleles?
- only variation between homologous chromosomes is in the alleles
- Homologous chromosomes are made of same genes in the same order
- There may be multiple alleles for a gene at the population level
- multiple alleles have a dominance hierachy
What is the function of epistasis?
- Where 2 non-linked genes interact, with one gene either masking or suppressing the other gene
- The alleles that are being masked are hypostatic to the epistatic alleles.
- Epistasis often involves a pathway where expression of one gene is dependent on the function of another gene.
What is recessive epistasis?
- Recessive epistasis is where the epistatic allele (the allele that masks another gene) is recessive.
- This means two copies of the epistatic allele must be present for expression of the hypostatic allele to be affected.
- In other words where two homozygous recessive alleles mask expression of another allele
What is dominant epistasis?
- Dominant epistasis is where the epistatic allele is dominant.
- This means only one copy of the epistatic allele must be present for expression of the hypostatic allele to be affected.
- In other words where one dominant allele masks expression of multiple other alleles
What is Chi-squared?
- Genetic diagrams are used to predict the expected phenotypic ratio of offspring.
- Predictions are rarely 100% accurate because of the random nature of gametes fusing during fertilisation.
- Chi-squared is used to compare observed phenotypic ratios with expected ratios.
- Chi-squared tells us if the difference between the observed and expected ratios are due to chance.
When is chi-squared used?
- when variation is discrete not continuous. This means the data are in categories (e.g. Aa and aa).
- when data show absolute numbers (whole numbers), normally frequencies.
- large sample size
- only raw count data allowed
- no data values equal zero
What is the null hypothesis?
- ‘There is no significant difference between observed and expected data, the difference is due to chance’.
- The chi-squared test is used to reject or accept the null hypothesis.
What is the equation for chi-squared?
X = Σ (O - E)^2/E
O = observed values.
E = expected values.
When do you accept the null hypothesis?
- If the chi-squared value is lower than the critical value so not a significant difference
- The difference between observed and expected data is due to chance
When do you reject the null hypothesis?
- If the chi-squared value is greater than the critical value so a significant difference
- The difference between observed and expected data is NOT due to chance.
How many alleles per gene do diploid organisms carry?
2
What is a dominant allele?
an allele whose characteristic will always appear in the phenotype, whether one or two are present
What is meant by a recessive allele?
an allele whose characteristic only appears in the phenotype if no dominant allele is present, meaning two must be pressent
What is meant by codominant alleles?
two dominant alleles that both contribute to the phenotype, either by showing a blend of both characteristics, or the characteristics appearing together
Define monohybrid inheritance.
where one phenotypic characteristic is controlled by a single gene
Define dihybrid inheritance
where two phenotypic characteristics are determined by two different genes present on two different chromosomes at the same time
What is meant by sex-linkage?
where an allele is located on one of the sex chromosomes, meaning its expression depends on the sex of the individual
Why are males more likely to express a recessive sex-linked allele?
most sex-linked alleles are located on the X chromosome. Therefore males only get one copy of the allele, so will express this characteristic even if its recessive. Since females get two alleles, this is less likely
Which parent do males inherit sex-linked characteristics from?
Their mother, since the Y chromosome can only come from their father. Therefore if the mother is heterozygous for sex-linked alleles, she is a carrier and may pass on the trait
