Inheritance Flashcards
Why did Mendel use peas?
- easy to grow
- can self-fertilise or cross fertilise
- flowers and fruit in the same year
- lots of seeds so next generations phenotype is statistically meaningful
Why did Mendel use characteristics such as yellow or green (or tall and dwarf) seeds to observe differences when breeding peas?
- characteristic controlled by single gene
- genes on different chromosomes
- clear cut so easy to tell apart
What is monohybrid inheritance?
Inheritance of a single gene
What is a test cross (back cross)?
cross between an individual with the phenotype of the dominant characteristic but unknown genotype with an indiividual that is homozygous recessive for the gene in question (from the offspring you can work out the genotype of the dominant individual)
What is codominance?
Both alleles of a gene are expressed, for example someone with the blood group AB has both A and B antigens on their RBC
What is incomplete dominance?
The heterozygote phenotype is intermediate between that of the two homozygotes e.g. pink is a mix between the red and white genotype on flowers
What is dihybrid inheritance?
Two different genes located on two different chromosomes
(always 9:3:3:1 if parents are heterozygous for both genes)
Meaning of progeny?
descendents/offspring
(to calculate it, divide the total number of that genotype by the number of homozygous recessive individuals e.g. 9:3:3:1)
What is a dihybrid test cross?
tests a genotype by crossing it with an individual that is homozygous recessive for both genes
What is meant by sex linkage?
genes on the sex chromosomes not on the autosomes
- when deciding phenotypes state gender as well as whether they have the disease
In autosomal linkage, what could explain a different ratio of phenotypes between offspring than that which is expected?
- crossing over at chiasmata can occur
- four type of gamete produced (but not in equal numbers)
What is autosomal linkage?
- Two different genes located on the same chromosome.
- dominant alleles of each gene are linked
- recessive alleles of each gene are linked
- any other genotype e.g. a dominant with a recessive is a result of crossing over at metaphase 1
What is a gene (point) mutation?
DNA isn’t accurately copied in S phase before cell division
- new codon may code for the same AA (code is degenerate) however if it doesn’t the structure and function of the protein may be effected
- e.b. sickle cell disease
What is a chromosome mutation?
chromosomes can become broken or damaged
- could effect a large number of genes
- chromosome does not rejoin accurately at the corresponding position on its homologous partner
What is an aneuploidy mutation?
Whole chromosomes or small number of chromosomes may be lost or added in a phenomenon called non-disjunction. (This is when chromosomes fail to separate to the poles of dividing cells at anaphase one or chromatids fail to separate at anaphase 2)
Offspring has an evene number of chromosomes so can’t form gamete in meiosis. For example Down Syndrome where person either has no or two copies of chromosome 21
What is meant by polyploidy?
Having more than two sets of chromosomes. This is common in plants and might have advantageous effects e.g. disease resistance. A defect in the spindle at meiosis means the number of chromosomes may double if the cell fails to divides following the first nuclear division after fertilisation
What is a carcinogen?
An agent that causes cancer
What is a tumour suppressor gene?
It suppresses the cell cycle (stops mitosis) so if this is inactivated uncontrolled cell division may occur
What is an oncogene?
It is the mutated version of a protooncogene, this promotes cell division. If an oncogene becomes permenantly activated there is uncontrolled cell division.
What is a benign tumour?
- slower growing
- doesn’t invade neighbouring tissues
- remains localised
maligant is opposite of that. Metatasises= cell spread in blood stream
What is epigenetics?
The control of gene expression by modifying DNA or histones but not by affeting the DNA nucleotide secquence
What does every gene have before the first exon?
a sequence of bases called the promoter region. This promotes the transcription of DNA as it allows transcription factor to bind. As more methyl groups bind to this region it means the regulator protein can’t attach so there is no gene expression as the gene is not transcribed. e.g. RNA Polymerase unable to attach
How are histones modified? How does this effect gene expression?
Addition of acetyl groups. As more histone groups are added the DNA is able to uncoil from around the histones. This makes the DNA more accessible so theres more transcription and gene expression increases.
What is always the null hypothesis?
There is no difference between observed and expected results
What significance level does biology work to
5%
Why might some phenotypes be more common than expected and others be rare?
Common phenotypes are to do with autosomal linkage, the rare phenotypes result from crossing over (recombinants)
What is the format for writing conclusion on X^2 test?
At ….. degrees of freedom and p=0.05, X2 value is less/more than critical value (state figure). This means less/more than 5% probability that the difference is due to chance so accept/reject the null hypothesis
What is the alternative hypothesis?
There is a significant difference between observed and expected results (of….)
What is endomitosis?
replication of chromosomes that is not followed by cytokinesis. If this happens in an early embryo, four sets of chromosomes are incoporated into the nuclear envolope and you produce tetraploid cells.
Why is polypolidy more common in plants?
- they can reproduce asexually
- they are hermaphroditic so do not use chromosomes to determine sex
What are the five types of point mutation?
- addition/ subtraction- base added/removed. This results in a frame shift mutation and all subsequent AA incorporated will be altered
- duplication- same base incorporated twice
- substitution- different base incorporated. If this is at a significant site on the protein molecule it may make a difference in activity. If an enzyme, the structure of the active site could be destroyed
- Inversion- adjacent bases on the same strand change position
Which types of genetic crosses do you need a capital letter with a superscript to represent?
Sex linkage, incomplete dominance and codominance
What is the SRY gene?
sex determining region on the y chromosome, it is able to switch on other genes genes on other chromosomes so male characteristics are expressed
What is meant by linked genes?
Genes are on the same chromosome so do not segregate independently at meiosis.
How do you recognise linked genes?
Numbers of progeny with different combinations of characteristics do not correspond with Medelian ratios for example 9:3:3:1 is a mendelian ration. Four genotypes but not in this ratio suggests the genes may be linked
Why might medelian ratios not be produced in the next generation due to linked genes?
Crossing over is rare so the majority of gametes should still be the parental BB or bb. So the number of gametes with different genotypes is not equal so medelian ratios are not produced in the next generation
What can determine sex?
- temperature eggs incubated at
- ploidy level (e.g. male bees are haploid)
- chromosome structure e.g. Male birds are ZZ but females are ZW
- Sequential Hermaphrodism- e.g. when dominant female clownfish dies, the dominant male changes sexes and takes her place.
