Flashcards in Inheritance Deck (42):
What is a gene?
A gene is a section of DNA that codes for the production of one polypeptide, e.g. the gene for fur colour in rabbits.
What are genetics?
The study of the way in which an organism inherits characteristics from its parent.
What is monohybrid inheritance?
Inheritance of a single characteristic, controlled by one gene (each parent has 2 alleles for the gene, one on each chromosome of a pair).
Greg Mendels pea experiment?
Investigated inheritance in peas because:
Easy to grow
Flowers self-fertilise and cross-fertilise.
Flowers are fruit and flowers in same year.
Make large number of seeds.
He chose 3 contrasting characteristics and found:
Controlled by single genes.
Controlled by genes on different chromosomes.
Clear cut and easy to tell apart.
What was Mendels first law called?
The law of segregation.
What does the first law say?
“The characteristics of an organism are determined by factors (alleles) which occur in pairs. Only one of a pair of factors can be present in a single gamete”.
What is an allele?
An allele is a version of a gene, e.g. black fur or white fur
What is a genotype?
It is the alleles that an organism has for a characteristic, e.g. BB, bb, Bb.
What is a phenotype?
It is the observable characteristic of an organism due to its genotype, e.g. back fur or white fur.
What is a homozygous organism?
If it only has one type of allele present in its genotype for that characteristic, e.g. BB or bb.
What is a heterozygous organism?
If it has two different alleles present in its genotype for that characteristic, e.g. Bb.
What is a dominant allele?
The one that shows itself in the phenotype of an organism whose genotype is heterozygous for that characteristic.
What is a recessive allele?
The one that does not show itself in the phenotype of an organism whose genotype is heterozygous for that characteristic.
P= parental generation
F1= first filial generation (offspring from p).
F2= second filial generation (offsping from f1).
What does a genetic cross show?
Genotypes of parents and offspring.
Phenotypes of parents and offspring.
Alleles present in the gametes.
Symbols for the alleles are defined.
Rules for a genetic cross??
Both parents donate 1 copy of a gene to each offspring.
Characteristic appearing in pehotype must have 1 allele in genotype.
Recessive characteristic is homozygous to be expressed in phenotype.
If 2 parents show charactistic not shown in offspring, both parents are carriers and offspring is homozygous recessive.
Instructions for writing a genetic cross?
1. Choose suitable symbols for alleles, eg. single letters, make sure upper and lower differ in shape+size.
2. Write parents genotypes with the appropriate pairs of letters.
3. Show gametes produced by parents, circle them.
4. Use a punnett square, to show possible combinations from random crossing.
5. Indicate the ratio of the phenotypes.
Test cross or back cross?
To tell is a dominant phenotype individual is heterozygous or homozygous, it may be crossed with the homozygous recessive.
If unknown is homozygous dominant, all offspring have dominant phenotype.
If unknown is heterozygous, half offspring have recessive and half will be dominant.
What is sex-determination?
Humans have 23 pairs of homologous chromosomes in diploid body cells.
23rd pair are the sex chromosomes, gender.
Other 22 pairs are AUTOSOMES.
Males have X and Y chromosomes, after gamete formation half sperm have X and half have Y.
What is co-dominance?
Both alleles are equally dominant so both alleles express themselves equally in the phenotype of a heterozygote.
Heterozygotes show a phenotype intermediate between those of the 2 homozygotes.
What is incomplete dominance?
The phenotype of the heterozygote is intermediate betwen the 2 parental phenotypes, rather than both being expressed, e.g. red and white makes pink.
Neither the allele for red flowers or white are dominant so no upper or lower case.
What are dihybrid crosses?
Looks at the inheritance of 2 characteristics (from 2 different genes), assumption is the 2 genes are carried on different pairs of homologous chromosomes and so independently sort during gamete formation.
What is dihybrid inheritance?
The inhertiance of two seperate genes each on a different pair of homologous chromosomes.
What is autosomal linkage?
Looks at the inheritance of 2 characteristics (from 2 different genes), genes are carried on the same pair of homologous chromosomes so cannot independently assort during gamete formation. Giving different ratios of offspring.
Said to be LINKED..
Types of linkage?
If linkage involves crossing over it is incomplete linkage.
If linkage does not involve crossing over it is complete linkage.
What is sex linkage?
Sex linked genes have their loci on the X chromosome.
In males Xy so only have one copy of the gene, even if recessive allele there is no 2nd to be dominant.
Females have 2 copies of the gene.
So sex linked recessive phenotypes are more common in men.
Cannot be passed from father to son.
Sex linkage and haemophilia?
Gene on X chromosome, blood clots slowly or not at all meaning people bleed.
Allele for normal is XH, and mutant version Xh.
XH XH Normal
XH Xh Carrier
Xh Xh Haemophilia
XH y Normal
Xh y Harmophilia
As a male only needs one mutant allele they are much more likely to get it.
It is sex-linked..
Inheritance of sex- linked conditions?
Males cannot pass onto sons as they need the y, but can onto daughters as give X only.
DMD = X- linked disease.
Normal is XD
Mutant is Xd
When carrier mum and normal dad crossed, daughter either carrier or normal and son either normal or affected.
When normal mum and affected dad crossed, daughter is carrier and son will never be affected as gets X from mum not dad.
What are pedigree diagrams?
The inheritance of a medical condition marked on to a family tree allows a geneticist to infer an explanation of how it is inherited. The diagram can indicate whether condition is sex linked and whether recessive or dominant gene.
How to analyse a pedigree diagram?
Need to know the genotypes, then it is possible to predict the probability of members of a further generation being affected.
Linkage, what is it?
When alleles on same chromosome, cannot move to opposite poles, move together = LINKED.
Parental is D+E or d+e.
Recombinant is d+E or D+e.
Crossing over is rare so most are parental phenotypes.
Mendalins ratio not produced, majority of offspring have parental phenotypes.
If the numbers of progeny with different combinations of characteristics do not correspond with mendalin ratios, it is likely that genes are linked. Normally why a null hypothesis is rejected.
How to calculate the crossover value (COV)??
Number of recombinants / number of progeny x100.
What is the Chi squared test and how carried out?
Testing if numbers of phenotype are close enough to the predicted values.
Mono = 3:1
Di = 9:3:3:1
1. Calculate expected values based on 9,3,3,1 (E).
2. For each phenotype calculate observed-expected (O-E).
3. Square O-E
4. Calculate (O-E)2 / E and add all values together.
The null hypothesis?
The null hypothesis always states that there is NO significant results between the observed results (ratio) and the expected results.
May be due to chance.
The alternative hypothesis?
Always states that there is a significant difference between the observed and expected results. The difference is due to something else, probably something biological happening and affecting the results. (could be due to linkage or crossing over).
What is mendel's second law?
'Law of independent assortment'
"Either one of a pair of contrasted characters may combine with either of another pair"
What happens if chi-squared is less than the 5% significance value?
The null hypothesis is accepted and alternative hypothesis is rejected.
We can they say that there is no significant difference between the observed and expected results (they fit the ratio).
What happens if the chi-squared value is greater than the 5% significance value?
The null hypothesis is rejected and the alternative hypothesis is accepted.
There is a significant difference between the observed and expected results. (don't fit ratio).
Differences between the, are NOT simply due to chance, something biological is causing them.
Degrees of freedom?
Measure of the number of values that can vary independently. In the crosses it is one less than number of classes of data, e.g. dihybrid cross is 4-1 = 3.
Constructing a conclusion?
Say whether chi squared value is less than critical value.
Say if it is equivalent to probability of 5%.
Say whether null hypothesis rejected or accepted.
Say at what level it is (5%).
If accepted say and deviation from predicted ratio is due to chance.