Inheritance Flashcards
Define Gene
A length of DNA on a chromosome normally coding for a specific polypeptide
Define Codominant
Alleles that are equally expressed in a heterozygote
Define Phenotype
The characteristics of an organism resulting from both its genotype and the effect of the environment
Define Genotype
The genetic make-up of of an organism e.g. the alleles
Define Monohybrid Inheritance
The inheritance of a single gene
What is the monohybrid ratio of phenotypes ?
3:1
What is mendels first law of heredity ?
Characteristics of an organism are determined by factors (genes) which occur in pairs. Only one member of a pair of factors can be represented in a single gamete
What is meant if a plant is described as true breeding?
It has been self fertilised for many generations, always producing the same phenotype. It has a homozygous genotype
What is the usual ratio for dihybrid inheritance ?
9:3:3:1
What is linkage ?
Genes occurring on the same chromosomes and therefore being inherited together, can be detected if expected ratio is not found in the offspring.
What is Mendel’s Second Law ?
Independent Assortment
Each of a pair of contrasted characters may be combined with either of another pair
When does crossing over take place?
During meiosis when homologous pairs of chromosomes come together during prophase 1 of meiosis, crossing over may occur between chromatids at points called chiasmata
What is the importance of crossing over?
If crossing over occurs between 2 genes, this separates alleles that were previously linked and allows them to combine in new recombinant genotypes. This is an important source of variation
What is chi-squared test used for?
To test the statistical significance of discontinuous (discrete) variables.
E.g. it can be used to determine if the results of a genetic cross are significantly different to expected results or whether the differences are due to chance alone
What is the null hypothesis?
A statement saying that any deviation from observed and expected results is due to chance alone.
E.g. no difference between the observed and expected results of the cross. If h test shows deviation is due to chance then it is accepted if not it is rejected.
What is independent assortment in inheritance and why is it so important ?
Explains why unlinked genes, found on different chromosomes, can combine to form all 4kind of gametes in dihybrid inheritance
Explain how monohybrid crosses are made
- Identify the characteristics and designate a letter to the dominant and recessive allele
- Give the genotype and phenotype of the parents
- Identify the gametes produced by both parents (TT and tt)
- Place the gametes in a punnet square and show all 4 possible crosses at fertilisation. Label each new genotype with the phenotype.
What is an example of codominance?
Blood typing
(AB - both A and B antigens on blood cells)
What is incomplete dominance ?
A blend of both alleles can be seen in the phenotype
E.g. Red, white and pink carnations
A cross between red and white - carnations produces an all pink F1
What is sex linkage?
Genes that are sex linked are found on the sex chromosomes (XX = Female, XY= Male)
Written as X (small letter) Y
What is Haemophilia?
A disease caused by a recessive allele of Factor 8 that does not code for the normal blood clotting factor.
For females to have a chance of inheriting the disease, both parents need to be carrying the recessive allele
Whilst males can inherit with only 1 recessive allele
What is duchenne muscular dystrophy ?
A progressive muscle disease caused by a sex-linked recessive allele that does not code correctly for the protein, dystrophin.
What is a mutation ?
A spontaneous, random change in a gene.
Where are mutations most likely to occur and in what organisms ?
Mutation rates are increased in organisms with short life cycles or frequent cell divisions.
Occur mostly during crossing over in prophase 1 and non-disjunction in anaphase 1 and 2
Describe advantageous mutations
Mutations affect protein synthesis and so change the phenotype of the organism. This leads to variation in species that causes evolution by natural selection
Describe disadvantageous mutations
Some genes called proto-oncogenes can mutate to become oncogenes, which are involved causing uncontrolled cell division to form cancer
What is gene point mutation and give an example
One mutated base in the DNA code for haemoglobin
So incorrect amino acid added into the polypeptide chain
Red Blood cell forms a sickle shape and is less efficient at carrying oxygen
E.g. Sickle cell anaemia
How is Down syndrome caused?
Chromosomal mutations
Failure of chromosome 21 to separate from its homologous pair during anaphase 1 in meiosis forms gametes with 2 copies and at fertilisation the zygote then has 3 copies of chromosome 21.
How isn’t the extra chromosome in Down syndrome fatal?
Normally an extra chromosome would be fatal to an organism by chromosome 21 is small, with only a few hundred genes, so the organism can survive
What is epigenetics?
The study of changes in gene expression without any changes to the DNA sequence
Every nucleus in cells of an organism contain a full set of genes, but in specialised cells only some genes are expressed, others are switched off
What do epigentic changes occur from ?
Diet
Drugs
Development
Aging
What is histone modification ?
Modification of histones means they can coil more tightly, preventing gene expression, or they can coil loosely allowing transcription and protein synthesis
DNA Methylation
DNA bases become methylated which reduces the transcription of the gene and so affects protein synthesis
What can epigentic changes cause?
Cancer
Autoimmune disease
Mental disorders
Diabetes
Define autosomes
A chromosome that is not a sex chromosome, in humans these are numbered 1-22
What is chromosome nondisconjunction?
Failure of homologous chromosomes to separate in meiosis 1 or sister chromatids to separate in meiosis II. Results in gametes with one extra or less chromosome than normal
What is a mutagen ?
A chemical,biological or physical agent that increases the rate of gene mutation above normal level
- Ionising radiation
What is a carcinogen ?
A type of mutagen that causes cancer
What is an oncogene?
Mutations of proto-oncogenes that are activated continuously, resulting in uncontrolled cell division
