Inheritance Flashcards
Define genotype
The alleles possessed by an organism
Define phenotype
The characteristics of an organism
What are the two modes of inheritance
A. Complete Dominance
B. Co-dominance
What is complete dominance
When the effect of one allele completely masks the effect of the other allele in a heterozygote
What is co-dominance?
When both alleles show their effect on the phenotype in the heterozygote
What is the notation used for co-dominance?
C^ B or I^A
How many blood groups are there?
There are four blood in the ABO system: A, B, AB, O
Which alleles in the ABO blood groups are co-dominant and which are recessive?
I^A and I^B are co-dominant and i is recessive
What are all the possible genotypes to all the phenotypes of blood groups?
Blood Type A I^A I^A or I^A i
Blood type B I^ B I^ B or I^B i
Blood Type AB I^A I^B
Blood Type O ii
What did Gregor Mendel’s experiments do?
They demonstrated the principles of inheritance using crosses of pea plants
What did Mendel do during his experiment?
- He crossed peas that had a different trait- the two varieties were true-breeding
- He examined the progeny of F1 gen. and found that all the offspring had the characteristic of one of the parents
- He self-fertilised the F1 gen. and examined the offspring of the F2 gen.
- There was a ratio of 3:1
What did Mendel’s success rely on?
On obtaining numerical results and using very large numbers of pea plants
What is a monohybrid cross?
A cross in which the inheritance of only one trait is studied
What is sex-linkage?
The gene controlling the characteristic is located on a sex chromosome
What is sex-linkage always due to?
Genes on the X because X has thousands of genes
Is X-linked recessive conditions more frequent in males or females?
Males
Is X-linked dominant conditions more frequent in males or females?
Females
Who can be carriers of recessive alleles and what does this mean?
Females can be carriers and so all sons inherit it from their mothers
What are genetic diseases due to?
They are due to specific alleles and can be inheritited
Describe cystic fibrosis
AUTOSOMAL RECESSIVE DISEASE
caused by allele of the CFTR gene coding for a chloride channel
Produce mucus which is thick and sticky- respiratory failure
Describe Huntington’s disease
AUTOSOMAL DOMINANT DISEASE
neurodegenerative disease caused by alleles of the gene HTT coding for Huntingtin, a protein. Symptoms include- dementia
hd= normal allele HD= Huntigton’s disease
Describe Hemophilia
X-LINKED RECESSIVE DISEASE
caused by alleles of genes that code for coagulation factors
blood clotting is impaired in patients
X^H X^h = normal, carrier
X^h Y= haemophiliac
What is red-green colour blindness?
X-LINKED RECESSIVE
mild disability
caused by allele coding for a photoreceptor protein that detects specific wavelengths
What is a test cross?
Used to identify if an individual with a dominant phenotype is homozygous or heterozygous. The individual that is under question is crossed with an individual that has the RECESSIVE PHENOTYPE.
How will the offspring be of a parent that is homozygous dominant?
ALL will have the dominant phenotype
Why do we sometimes get different ratios in monohybrid crosses?
- Number of offspring is too small
2. One of the resulting genotypes of the offspring is lethal
What are lethal alleles?
Alleles that cause an organism to die only when present in a homozygous condition.
Thus causing the ratio to be 2: 1 instead of 3: 1
What is the law of segregation?
Two alleles of a gene separate into different haploid gametes in meiosis
What is the law of independent assortment?
Presence of one allele of one gene in a gamete has no influence over which allele of the other gene is present in the gamete
STANDS FOR UNLINKED GENES
What is independent assortment due?
Random orientation of bivalents in metaphase I of meiosis
What are unlinked genes?
Genes that assort independently
If the chromosomes are unlinked, what are the gametes produced by a genotype AaBb
AB, Ab, aB, ab
What is gene linkage?
Genes whose loci are on the same chromosome and do not follow the law of independent assortment–> inherited together
New combination can be produced ONLY if crossing over takes place
Show how we demonstrate linked A and B genes
a b
What is Mendel’s dihybrid cross?
2 characteristics/traits examined
What happened in Mendel’s dihybrid cross?
Performed a test cross between two characteristics: seed shape and seed colour.
R (dominant, round) and r (recessive and wrinkled) seed colour is controlled by Y (dominant, yellow) and y (recessive, green)
What is the result of a dihybrid cross between two heterozygotes?
9:3:3:1 ratio
What is the result of a cross between a double homozygous recessive and a heterozygote?
1:1:1:1 ratio
Who proposed gene linkage and how?
Morgan proposed it.
He found patterns of inheritance that differed between males and females
How can non-mendelian ratios be observed?
- The genes are linked
- Either of the genes has co-dominant alleles
- Either of the genes is sex-linked
- There is epistasis
- A genotype is lethal
- Polygenic inheritance
What are recombinants?
Individuals that have a different combination of characters from parents
How can new combinations arise in gene linkage?
only if crossing over takes place between non-sister chromatids
When genes are linked, what happens when two double heterozygotes are crossed?
There are more of the parental combinations and fewer of the recombinants in the progeny
What is a chi- squared test?
A test to see whether there is a statistically significant difference between the observed and expected results
What are the steps for the Chi- Squared test?
- Define the hypothesis:
Null hypothesis- there is no difference
Alternative hypothesis- there is a difference - Calculate the expected frequencies for each using:#
expected frequency= expected probability * actual total - Calculate x^2 using:
x^2= Sum( fo - fe)^2 /fe fo= observed frequency, fe= expected frequency - Find the degrees of freedom (one less than the total number of phenotypic classes)
- compare x^2 with critical value for specific degrees of freedom and p- level
- If x^2> critical —> reject null
If x^2< critical—-> accept null
What is a pedigree chart?
A chart of the genetic history of a family over several generations
What is a pedigree chart used for?
To deduce the pattern of inheritance of a characteristic, almost always of a genetic disease
In a pedigree chart, how can you tell if a disease is autosomal dominant?
- If 2 affected parents have an unaffected offspring
- ALL affected individuals MUST have at least one affected parent
- If both parents are unaffected, all offspring must be unaffected
In a pedigree chart, how can you tell if a disease is autosomal recesssive?
- If 2 unaffected parents have an affected offspring
2. If both parents show a trait, all offspring must exhibit the trait
In a pedigree chart, how can you tell if a disease is X-linked dominant?
- if a male shows a trait, so too must all his daughters as well as his mother
- An unaffected mother cannot have affected sons
- More common in females
In a pedigree chart, how can yo tell if a disease is X-linked recessive?
- If a female shows a trait, then all her sons must as well including her father
- An unaffected mother can have affected sons if she is a carrrier
- more common in males
What is epistasis?
Interaction between genes
This is where the effect of one gene is dependent on the presence of a one or more ‘modifier genes’.
What is variation?
The differences between members of the same species
What are sources of variation?
Gene mutations
Meiosis
Sexual reproduction
What is discrete variation?
Individuals that fall into a number of distinct categories and it is usually due to one gene
What is continuous variation?
Individuals can have any level of the phenotype between two extremes
This is genetically-determined due to polygenic inheritance
What is polygenic inheritiance?
When a characteristic is controlled by the combined effects of 2 or more genes
Also influence by environmental factors
What are histograms used for?
To show the expected distribution of the phenotypes in the offspring of a cross when a single trait is controlled by multiple genes
As the number of genes increase, the distribution becomes increasingly closer to the normal distribution
