4.3 inheritance Flashcards
What is genetics?
The study of mechanisms by which an organism inherits characteristics from its parents
What is the difference between a gene and an allele?
A gene is a section of DNA found at a specific position on a chromosome (locus) that codes for a particular polypeptide
Alleles are versions of the same gene which are found at the same locus on homologous chromosomes
What is the difference between a heterozygous and homozygous genotype?
In a heterozygous genotype the alleles for a given gene are different (e.g. Hh)
In a homozygous genotype the alleles for a given gene are the same (e.g. HH or hh)
What is Mendel’s first law of inheritance?
The law of segregation
Only one of a pair of alleles is present in the gamete
(Each gamete contains only one chromosome from each homologous pair)
What is the difference between a recessive and a dominant allele?
A dominant allele is always expressed within a genotype
A recessive allele is only expressed if both versions of the allele are recessive; it is masked by a dominant allele
What is meant by the term phenotype?
The observable characteristic of an organism
What are codominant alleles?
Alleles which are expressed equally in a phenotype
e.g. blood group AB has both A and B alleles
What is Mendel’s second law of inheritance?
Either member of an allelic pair may combine randomly with either of another pair (provided the genes are not linked) due to independent assortment during metaphase I
What is the ‘F2’ generation in a genetic cross?
The generation produced as a result of selfing two heterozygous individuals from the F1 generation
What are the expected phenotypic ratios within the F2 generation following a monohybrid cross (Yy x Yy)?
3:1
3 with dominant characteristic, 1 with recessive characteristic
What is the ‘F1’ generation in a genetic cross?
The first generation produced as a result of breeding two homozygous individuals, one with the dominant characteristic(s), the other with the recessive characteristic(s)
All offspring are heterozygous, with the dominant trait(s)
What is a linked gene?
Genes located close together on the same chromosome that tend to be inherited together
What are the expected phenotypic ratios within the F2 generation following a dihybrid cross (YyRr x YyRr)?
9:3:3:1
9 with both dominant characteristics, 3 with either dominant characteristic, 1 with both recessive characteristics
What do Mendel’s laws of inheritance assume with regard to dihybrid inheritance?
The genes are not linked and are located on different chromosomes
How is a test cross carried out?
Cross the organism with the dominant characteristic with an individual with a recessive genotype.
If all of the offspring have the dominant characteristic, the unknown parent is homozygous.
If only 50% of the offspring have the dominant characteristic, the unknown parent is heterozygous
Why are test crosses carried out?
To determine whether a dominant characteristic observed in an organism is determined by one or two dominant alleles (homozygous dominant or heterozygous)
State the expected results if a dihybrid test cross is carried out on a doubly heterozygous individual (YyRr x yyrr)
The offspring show a phenotypic ratio of 1:1:1:1
Explain why the expected phenotypic ratio from a dihybrid test cross between a doubly heterozygous individual (YyRr) and a doubly recessive individual (yyrr) may not be 1:1:1:1
This is due to crossing over of linked genes in prophase I
The recombinants will be present in lower numbers
In dihybrid inheritance of linked genes, state the phenotypic ratio which is displayed from a cross between two doubly heterozygous individuals
It shows a 3:1 ratio not the expected 9:3:3:1 ratio shown with unlinked genes
Draw a diagram showing crossing over of linked genes in prophase I, between two individuals, one doubly homozygous dominant and the other doubly recessive
What test can be carried out to investigate if the difference between expected and observed results from a dihybrid cross is due to chance alone or a significant event?
Chi-squared
Mendelian genetics is based on the assumption that genes are not linked.
Significant deviation from the expected results suggests that genes are linked
How are the ‘degrees of freedom’ calculated?
The number of categories minus one (n-1)
In this case, degrees of freedom = 3 (4-1)
How are chi-squared results interpreted?
If the chi-squared value is greater than the critical value (at 0.05 probability) then the null hypothesis is rejected.
The difference between observed and expected results is significant and not due to chance alone
In a dihybrid test cross this suggests genes are linked
What are sex chromosomes known as?
Autosomes
What is sex-linked inheritance?
Inheritance of genes found on the sex chromosomes (autosomes)
Why are most sex-linked disorders linked to the X chromosome?
It is much longer than the Y chromosome and contains far more genes
Name two sex-linked disorders carried on the X chromosome
Duchenne muscular dystrophy (DMD)
Haemophilia
Who can be a carrier of a sex-linked disorder on the X chromosome?
Only the mother as she has two copies of the gene (XX)
The father will be affected as he only has one copy of the X chromosome (XY)
Explain why predominantly males are affected by X linked disorders
Males: XY, females XX
If the male inherits the recessive allele from their mother (X chromosome), they have no other copy of the gene and so will inherit the disease
If females inherit the recessive allele from one parent, they will still have one normal copy of the gene
Why is haemophilia relatively rare in the female population?
The female would have to be doubly recessive to have the condition
Is a man able to pass on the haemophilia recessive allele to his son?
No. He can only pass the Y chromosome onto his son
What is a mutation?
A spontaneous, random change in a gene
What are the two types of mutation?
Chromosomal and gene mutations
Gene (point) mutations affect single bases in a gene
Chromosomal mutations affect many genes
What is a gene point mutation?
A change to a single nucleotide within DNA
In which organisms are mutation rates increased?
Those with short life cycles and frequent cell division
e.g. bacteria
What are the 3 types of gene point mutation?
Silent mutation - same amino acid is coded for
Missense mutation - a different amino acid is coded for
Nonsense mutation - the code is change to STOP
What is the consequence of a missense mutation?
An alternative amino acid is inserted into the polypeptide chain.
The primary structure is changed, thus potentially altering the secondary and tertiary structures, which will modify the protein’s function.
Are all mutations harmful?
No - some are neutral and some are beneficial
Beneficial mutations drive evolution, by introducing phenotypes which confer a selective advantage
When do mutations occur most frequently?
Meiosis - crossing over in Prophase I
- non-disjunction in Anaphase I and II
What type of mutation gives rise to sickle cell anaemia?
A missense gene point mutation in the gene for the haemoglobin beta chain, altering the quaternary structure
The misfolded haemoglobin causes the red blood cells to have a sickle cell shape. They are less efficient at carrying oxygen and the red blood cells can clump together in the capillaries causing clots and oxygen deprivation
Which type of mutation gives rise to Down’s syndrome?
Non-disjunction of the chromosome pair 21
When can non-disjunction occur during meiosis?
Anaphase I - non-separation of a homologous pair of chromosomes
Anaphase 2 - non-separation of sister chromatids
In Down’s syndrome, non-disjunction generally occurs during Anaphase I. Does this occur before or after puberty?
After puberty - during maturation of the secondary oocyte prior to ovulation
Before puberty the secondary oocyte within the ovary is suspended at Prophase I, so will not have undergone Anaphase I yet.
Prior to ovulation a secondary oocyte matures and progresses to Metaphase II, having undergone Anaphase I.
Are there more gametes affected by non-disjunction in Anaphase I or Anaphase II?
Anaphase I - every gamete will be affected
(In Anaphase II - half of the gametes are affected)
What is epigenetics?
The study of changes in gene expression without any changes to the DNA sequence
What are the main epigenetic changes?
DNA methylation - a methyl group is attached to a DNA base, preventing transcription of the gene and protein synthesis
Histone modification - the proteins around which the DNA is coiled can be modified to coil the DNA more tightly, preventing gene expression, or more loosely to increase gene expression
Give examples of mutagens, which increase the rate of mutation.
Ionising radiation - gamma radiation, UV light, X-rays
Chemicals including polycyclic hydrocarbons in cigarette smoke
What is a carcinogen?
A mutagen which causes cancer
What is a proto-oncogene?
A gene that regulates cell division.
If this is mutated, it will become an oncogene, causing uncontrolled cell division and cancer
