Genetics Flashcards
Define a gene.
A sequence of D.N.A bases on the coding strand of a D.N.A molecule responsible for the production of a polypeptide chain.
Define a locus.
the position of a gene on a chromosome
Define allele.
one version of a gene at a particular locus
Define alleles
a pair of genes at the same locus on two homologous chromosomes controlling the same feature.
Define homozygous.
a pair of identical alleles e.g AA or aa
Define heterozygous.
a pair of contrasting alleles e.g Aa
Define genotype
the genetic makeup of an individual. E.g AA or aa or Aa
Define phenotype
the outward appearance created by the genotype.
Define dominant.
an allele which shows 100% of its phenotype in the heterozygous form.
Define recessive.
an allele which shows 0% of its phenotype in the heterozygous form.
Define Monohybrid inheritance.
The inheritance of a single characteristic which can take two different forms e.g tall and short in pea plants.
What Experiments did Mendel perform to discover Monohybrid Inheritance?
Mendel carried out a series of Experiments on pea plants. The first feature he investigated was height. He found this took only two forms - tall and short (dwarf). He then carried out the following crosses:
Tall X Short → Tall (F1 generation)
And then crossed the F1 generation with itself.
F1 X F1 → Tall and Short in the ratio 3:1
What Is blending?
This incorrect theory preceded Mendel’s work and suggested that inheritance occurred through the blending of the ‘factors’ (genes) from the maternal and paternal side.
What conclusions can be drawn from Mendel’s experiments on Monohybrid inheritance?
That the blending theory of inheritance is incorrect, i.e they remain apart or discrete.
Factors (genes) for inheritance are either dominant or recessive.
The factors controlling inheritance must occur in pairs.
When gametes are formed only one pair of factors (genes) enters each gamete.
What evidence do Mendel’s experiments give in order to suggest that the factors controlling inheritance must occur in pairs?
The F1 generation is tall, therefore it must have a factor (gene) for tall. However F1 x F1 produces tall and short so the F1 generation must also contain a factor (gene) for short.
What is Mendel’s first law (also known as the first law of genetics)?
Pairs of genes are split apart during meiosis in gamete formation.
What ratio of offspring in the form Dominant:Recessive does monohybrid inheritance give?
3:1
Define Dihybrid Inheritance.
The Inheritance of two features, both of which can take two forms. e.g Mendel looked at Tall pea plants with green seeds crossed with dwarf plants with Yellow seeds.
What ratio does a Dihybrid cross produce?
9:3:3:1
What is the law of independent assortment?
When two features are controlled by two contrasting genes, each one of each pair can enter the gamete with one of the other two
What is independent assortment responsible for generating?
This process generates huge amounts of genetic variation in offspring. We now know that it is the result of the arrangement of chromosomes around the equator in metaphase II of meiosis.
What is linkage
linkage is a circumstance where independent assortment breaks down and a dihybrid cross does not produce a 9:3:3:1 phenotype ratio.
What causes linkage?
Linkage results from both alleles being carried on the same chromosome. Because of this they travel together into a gamete more often than the normal situation in which they are on different chromosomes
What is incomplete linkage?
This is when two genes are on the same chromosomes but are some distance apart.
In the case of incomplete linkage, how are new gene combinations produced?
By crossing over only. (Independent assortment has no bearing here)
What is complete linkage?
This is the situation in which two pairs of alleles are carried on the same chromosome but with no distance between them. This makes it impossible (very very very unlikely) that crossing over will occur.
What is co-dominance?
This is the idea that two alleles in the heterozygous situation can show 100% of their feature.
Give two examples of Co-Dominance.
the genes A and B in human blood groups and the genes R and W in snapdragon flowers.
Give an example of multiple alleles.
Human blood groups. in this case there are three alleles - A, B and O
What does having multiple alleles for a characteristic result in?
This results in there being many more phenotypes than when there were only two alleles.
What is sexual inheritance?
The inheritance of gender.
What two chromosomes are responsible for sexual inheritance?
The X and Y chromosomes
Are the X and Y chromosomes homologous?
No, they’re non-homologous
In humans and mammals, what combination of X and Y chromosomes results in a male phenotype?
XY
In humans and mammals, what combination of X and Y chromosomes results in a female phenotype?
XX
What sex chromosomes can female gametes contain, and what is the technical term for this?
Female sex chromosomes are always X - they are known as being homogametic.
What sex chromosomes can male gametes contain, and what is the technical term for this?
50% of male gametes have the X chromosome and 50% of male gametes have the Y chromosome - they are known as being heterogametic.
Based on the idea that males are heterogametic and females are homogametic, which gametes determine the sex of the next generation?
The male gametes (XY)
Why are roughly 50% of the population male and 50% female?
Because of random fertilization.
Define Sex linkage.
Any genetic condition found more often in one sex than the other. (Normally found more in males)
Give some examples of sex-linked conditions.
Haemophilia and colour blindness.
Why do males more often show sex linkage than females?
Normally, males show sex linkage rather than females because they are XY not XX. Therefore males only require one Recessive allele on the X chromosome to give a recessive phenotype whilst females require two.
What are the two main types of mutation?
Gene mutation
Chromosome mutation
What are gene mutations and when do they occur?
Gene mutations are copying errors during D.N.A replication at the end of interphase.
What are chromosome mutations and when do they occur?
Chromosome mutations involve whole chromosomes or parts of a chromosome which are either gained or lost. They occur much more commonly in meiosis than mitosis. Because meiosis is more complex that mitosis.
Name some types of chromosome mutations.
Non-disjunction (most common).
Deletion - section of chromosome lost during crossing over
Translocation - chromosome section joins a non-homologous chromosome.
Inversion - chromosome section rejoins upside down) and duplication (chromosome section is copied by mistake
Describe non-disjunction.
This is the result of the failure of chromatids to separate in anaphase II of meiosis I.e both chromatids move to the same pole. This results in a gamete with one extra chromosome and a second gamete with one chromosome missing.
Give an example of non-disjunction in humans.
In humans non-disjunction in chromosome number 21 produces a gamete with an extra 21st chromosome I.e 24 chromosomes in one gamete. This then combines with a normal gamete carrying 23 chromosomes. The result is a zygote containing 47 chromosomes. I.e 3 copies of chromosome 21. This results in Down’s syndrome which produces slow mental and physical development.
Why do many gene mutations have no effect on the phenotype?
Most of these errors have no effect on the phenotype because they occur in non-functional genes, in introns or because The genetic code is degenerate.
What is a substitution gene mutation?
This is when one or more D.N.A base is copied wrongly
Give an example of partial dominance.
The two alleles involved in sickle cell anaemia
Hb shows 60% of its phenotype and HbS shows 40% of its phenotype in the heterozygous form.
What is a mutagen?
A mutagen is anything which speeds up the rate of mutation.
What is a carcinogen?
Carcinogens are also mutagens but they result in copying errors in genes called oncogenes. These are genes which are involved in cell replication speed cell death.
What two criteria define a cancer cell?
a cell which divides very rapidly and will not die.
