6.2 Flashcards
What is Discontinuous Variation?
discontinuous variation is genetic variation where phenotype classes are distinct and discrete.
There are no or very few intermediates (two or more non overlapping categories)
What are examples of Discontinuous Variation?
- Sex
- Blood group
- Ear lobes
What controls Discontinuous variation?
Characteristics that exhibit discontinuous variation are usually determined by the alleles of a single gene locus.
They are Monogenic (determined by a single gene)
In either case:
- Different alleles at a single gene locus have large effects on the phenotype
- Different gene loci have quite different effects on the characteristics
What is Continuous Variation?
Continuous Variation is variation that produces phenotypic variation where the quantitative traits vary by very small amounts between one group and the next
where a population shows a range with a smooth gradation between many intermediates
What are examples of Continuous Variation?
- Eye colour
- Height
- Skin colour
- Hair colour
- Foot size
What controls Continuous variations?
characteristics that exhibit continuous variation are usually determined by many genes
They are polygenic (many genes involved in determining characteristics)
The alleles of each gene may contribute a small amount to the phenotype, thus the alleles have an additive effect on the phenotype.
The greater the number of gene loci determining a characteristic, the more continuous the variation (the greater the range)
what is quantitative genetics
quantitative genetics is the study of genetics of such inherited characteristics. Plant breeders use this knowledge as crop plants are polygenic
what is a genotype
genotype is the genetic makeup of an organism
what is a phenotype
phenotype is the visible characteristics of an organism
what two factors is a phenotype influenced by
influenced by its environment and its genotype
what is a mutation
a change to genetic material, this may involve structures to the DNA or changes to the gross structure of the chromosomes
what are mutagens
mutagens are certain physical and chemical agents that can increase the rate of mutation
What are the three main Mutagenic Agents that cause Gene Mutations?
- Physical Agents
- Chemical Agents
- Biological Agents
What Physical agents can cause Gene Mutations?
- X-rays
- Gamma Rays
- UV light
What Chemical agents can cause Gene Mutations?
-Benzopyrene (In tobacco smoke)
- Mustard Gas
- Nitrous Acid
- Aromatic Amines
- Free Oxygen Radicals
- Colchicine
What Biological Agents can cause Gene Mutations?
- Some Viruses
- Viral Nucleic acid incorporated into our genome
- Food Contaminants
What are the characteristics of Mutations that arise from Gamete Formation?
-harmful, advantageous or neutral
Mutations that occur during Gamete formation are also:
- Persistent: they can be transmitted through many generations without change
- Random : they are not directed by a need on the part of the organism in which they occur
What is an Allele?
An allele is a version of a gene.
What does Heterozygous mean?
Heterozygous means having different alleles at a particular gene locus on a pair of homologous chromosomes.
What does Homozygous mean?
Homozygous means having identical alleles at a particular gene locus on a pair of homologous chromosomes.
What does Monogenic mean?
Monogenic means determined by a single gene.
What does Dihybrid mean?
Dihybrid means involving two gene loci
who layed out the foundation of genetics
Gregor Mandel (experimenting with pea plants)
What is the expected phenotypic ratio of crossing heterozygous parents in a Monohybrid cross?
3:1 is a Monohybrid Phenotypic ratio.
how can we know the Genotype of Phenotypically similar individuals?
Some tall plants will be heterozygous (Tt) and some will be homozygous dominant (TT).
To know which one a plant is, we would cross each one with a homozygous recessive (tt) small plant.
If any of the offspring were small plants, this would indicate the unknown genotype to be Tt. If all the plants were tall, this would indicate the unknown genotype to be TT.
What did Mendel conclude about Dihybrid inheritance?
Mendel concluded that:
- The alleles of the two genes are inherited independently of each other, so each gamete has one allele for each gene locus
- During fertilisation, any one of an allele pair can combine with any one of another allele pair.
what was the genotype and phenotype when mendel bred a green wrinkled pea with a yellow round seeded pea
a yellow round seeded pea
YyRr
What is the expected phenotypic ratio of crossing heterozygous parents in a Dihybrid cross?
(Assume that colour and seed shape are two monogenic characteristics )they do not influence one another)
The expected phenotypic ratio of crossing heterozygous parents would be 9:3:3:1 in which the alleles of two different genes assort independently into gametes.
what would it suggest about the chromosomes if a 9.3.3.1 ratio was produced
when investigating dihybrid inheritance, Mendel without knowing about genes or the process of meiosis, chose two characteristics, and the genes for which are on different chromosomes.
If the two genes were on the same chromosome, the inheritance pattern would be different .
What is Codominance?
Codominance is where both alleles present in the genotype of a heterozygous individual contribute to the individuals phenotype.
What does Multiple Alleles refer to?
Multiple alleles refer to characteristics for which there are three or more alleles at a specific gene locus in the population’s gene pool.
However, individuals can only possess two alleles, one on each gene locus, in a pair of homologous chromosomes. as humans are diploid
what is an example of codominance
ABO Human Blood groups
sickle cell anaemia
How is ABO Human Blood groups an example of Codominance and Multiple alleles?
The four blood groups are determined by three alleles of a single gene on chromosome 9.
This gene encodes for an isoagglutinogen on the surface of erythrocytes.
A and B are both dominant to O, which is recessive.
A and B are codominant, so can be expressed together
any individual will only have two of the 3 alleles within their genotype
How is Sickle Cell Anemia an example of Codominance and Multiple alleles?
Sickle cell anemia is caused by a mutation in the gene that codes for the B-globin chain of Hemoglobin.
The mutant allele is given the symbol Hb^s and the normal is Hb^N
In heterozygous people, half the Hemoglobin in their red blood is normal and half is abnormal. However, heterozygous people do not suffer from Sickle cell Anemia.
If we consider the type of Hemoglobin as the phenotype, then the alleles are considered codominant, but if we take sickle cell anemia as the phenotype, there is one dominant allele and one recessive allele.
How is Cow skin colour an example of Codominance?
Coat color in shorthorn cattle is an example of Codominant inheritance. The one gene for coat colour has two alleles. C^r (red) and C^w (white)
Cattle that are homozygous for red or white exhibit that colour. (C^rC^r) OR (C^wC^w)
Cattle that are heterozygous have both red and white hairs. (C^rC^w)
How is Plant colour an example of Codominance?
Plant color is determined by two alleles.
Plants that are homozygous for red or white exhibit that colour.
Plants that are heterozygous have both red and white colour.
how is blood group AB shown in offspring genotypes
shown as I^BI^A
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a
learning tip: when we refer to multiple alleles and say that a particular gene has more than 3 alleles, we mean within the gene pool of the population or species. E very individual organism has, within their genome, only two of these alleles at a particular gene locus
What is Sex-linkage?
Sex-linkage is where a gene is present on one of the sex chromosomes, but not the other.
What determines Human Sex?
In humans, sex is determined by one of the 23 pairs of chromosomes, called the sex chromosomes.
What are Autosomes?
Autosomes are the remaining 22 chromosomes that are not sex chromosomes. Each of the autosomal pairs is fully homologous.
They match for length and contain the same genes at the same loci
Are the X and Y chromosomes in males homologous?
The X and Y chromosomes are not fully homologous.
Only a small part of one matches a small part of the other so that these chromosomes can pair up before meiosis.
what are the sex chromosomes in females
XX
what are the sex chromosomes in males
XY
What is the responsibility for the genes located on an X chromosome?
The X chromosome contains over 1000 genes that are involved in determining characteristics or metabolic functions.
They are not concerned with sex determination, and most of these have no partner alleles on the Y chromosome.
If a female has one abnormal allele on one of her X chromosomes, she will probably have a functioning allele of the same gene on her other X chromosome.
Why are males more susceptible to genetic diseases?
If a male inherits, from his mother, an X chromosome with a genetically abnormal allele for a particular gene, he will suffer from a genetic disease, as he will not have a functioning allele for that gene.
Males are functionally haploid, or hemizygous, for X-linked genes. They cannot be heterozygous or Homozygous for X-linked genes.
What are examples of sex linked characteristics
Hemophillia A and Colour blindness
What is Hemophillia A and how is it Sex-linked?
A person with Haemophilia A is unable to clot blood fast enough.
One of the genes on the non-homologous region of the X chromosome codes for a blood-clotting protein called factor 8. A mutated form of the allele codes for nonfunctioning factor 8.
A female with one abnormal allele and one functioning allele produces enough factor 8 to enable her blood to clot normally when required. However, this female is a carrier for the disease.
If such a female passes the X chromosome containing the faulty allele to her son, he will have no functioning allele for factor 8 on his Y chromosome.
As a result, he will suffer from haemophilia A.
what would the genotype look like for a male with haemophillia
X^h Y
what would a symptomless female look like who is recessive for hemophilia
X^H X^h
What is Colorblindness and how is it Sex-linked?
One of the genes involved in coding for a protein involved in colour vision is on the X chromosome, but not on the Y chromosome.
A mutated allele of this gene may result in colour blindness - an inability to distinguish between red and green.
A female with one abnormal allele and one functioning allele will not suffer from colour blindness, but a male with an abnormal allele on his X chromosome will not have a functioning allele on his Y chromosome and will therefore suffer from red-green colour blindness.
How is coat colour Sex-linked in cats?
One of the genes for coat colour in cats is sex-linked. It is on the non-homologous region of the X-chromosome.
The alleles O (organge) and B(black) are codiminant meaning cats with XCO XCB will appear a mixture of orange and black.
Male cats can only be one colour as they only have one X chromosome
Do females have twice as many X-linked genes?
In every female cell nucleus, one X chromosome is inactivated.
Determination of which member of the pair of X chromosomes becomes inactivated is random and happens during early embryonic development
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