Genetic Information Flashcards
What are the two ways in which meiosis introduces genetic variation?
Independent assortment (random assortment) and crossing over (recombination)
What is independent assortment?
In meiosis the components of the chromosome pairs from the father and the mother are distributed into the gametes randomly. So in humans any number from none to all 23 chromosomes in your gametes could come from either your maternal or paternal chromosomes. Independent assortment results in many new combinations or alleles and introduces genetic variation
What is crossing over?
This process takes place during meiosis when large multi-enzyme complexes cut and join bite of the maternal and paternal chromatids together at the chiasmata. This exchange of genetic material leads to added genetic variation as many new combinations of alleles arise. Crossing over is also a potential source of mutations
What is a mutation?
A permenant change in the DNA of a whole organism
What do the majority of mutations result in?
Slight changes which have little or no effect on the organism
How can mutations be the source of an evolutionary change?
If environmental conditions change some mutations may make an organism better adapted to survive
What are the three main types of mutation and describe these?
• point of gene mutations that are caused by the miscopying of just one or a small number of nucleotides. They include:
- substitutions, where one base is substituted for another
- deletions where a base is completely lost in the sequence
- insertions when an extra base is added in that is a repeat of one of the bases already there or a different base entirely
• chromosomal mutations involve changes in the position of genes within the chromosomes. These are more likely to make a measurable change in an organisms than a gene mutation
• chromosome mutations occur when an entire chromosome is lost during meiosis or duplicated in one cell by errors in the process. These usually have a major impact on the organism
How is random fertilisation a source or genetic variation?
In sexual reproduction the male and female gametes from two unrelated individuals fuse to form a new genetic individual. This introduces considerable genetic variation into the offspring. There are usually many more male gametes than female gametes and the combination of the two to fuse to form a new individual is completely random
What are rhe three main ways in which genetic variation is introduced?
- meiosis
- mutations
- random fertilisation
What is the phenotype of an organism?
The physical and chemical characteristics that make up the appearance of an organism
What is the phenotype partly the result of?
The genotype (genetic information) passed from parents to their offspring and partly the effect of the environment in which the organism lives.
What do chromosomes in homogolous pairs carry that are the same?
They carry the same genes
Where is the gene for a particular characteristic always found?
In the same position or locus
How do each gene exist in the two chromosomes in a homogolous pairs?
In slightly different versions called alleles (variants) e.g. one allele may code for a tall plant and one may code for a dwarf plant
What is a heterozygote?
An individual where two alleles coding for a particular characteristic are identical
What is a heterozygote?
An individual where the two alleles coding for a particular characteristic are different
What does it mean when a phenotype shows dominance?
It is expressed whether the individual is homozygous for the characteristic or not
What does it mean when a phenotype is recessive?
It is only expressed when both alleles code forbthe recessive feature (when the individual is homozygous recessive for that trait)
In genetic diagrans how are the alleles coding for dominant phenotypes usually represented and how are those coding for the recessive feature represented?
The dominant alleles are represented by a capital leter and the recessive alleles are represented by a lower case version of the same letter
What are polygeneic phenotype traits determined by?
Several interacting genes
Why are homozygotes referred to as true breeding?
Because if you cross two individuals that are homozygous for the same characteristic, all the offspring of all the generations that follow will show this same characteristic in their phenotype (unless a mutation occurs)
What is a monogenic cross?
A genetic cross where only one gene is considered
How can we represent monogenic crosses?
Using punnet squares
What is the first generation of a monogenic cross called?
The first fillial generation. They will all have the same genotype for the characteristic and be heterozgous
What is the second generation of a monogenic cross called?
F2 the second fillial generation
What is a test cross and what is it used for?
When a homozygous dominant individual or heterozygous individual are crossed with a homozygous recessive individual to find out whether the individual is homozygous dominant or heterozygous. This reveals the parental genotype
Why are the numbers never precise in real genetic experiments in terms of the theoretical ratios?
- chance plays a large role in reproduction. The combination of alleles in each gamete is completely random as is the joining of particular gametes, unlike the theoretical diagrams
- some offspring die before they can be sampled, e.g. some seeds do not germinate and some embryos miscarry
- inefficient sampling techniques (e.g. something can escape)
How can you reduce the sampling error in a genetic test?
By using a larger sample size
Who is the genetics we study today based on the work of?
Gregor Mendel
Who is the genetics we study today based on the work of?
Gregor Mendel
Why did mendel study peas?
- Because he could see that they had characteristics (the colour and shape of the peas and the pods and the height of the plants) that varied in a clear cut way.
- they were also easy to grow and he could control what plants pollinated each other
What is the law of segregation?
It describes Mendels firsr law which states that one unit/allele for each trait is inheritted from each parent to give a total of two alleles for each trait. The segregation (seperation) of alleles in each pair takes place when the gametes are formed and some alleles code for phenotypes that are dominant over others
What is Medels law of independent assortment?
His second law. It states that different traits are inherrited independently of each other. This means that the inheritance of alleles for one phenotype has nothing to do with the inheritance of alleles for another characteristic
What are the exceptions to the independent assortment law?
Gene linkage and polygeneic inheritance
What does it mean if a gene has multiple alleles?
It means there are more than two possible variants at a particular locus
Describe the codominance and multiple alleles of the human ABO blood group?
- In the human ABO blood group system there are three possible alleles - A, B and O. These are usually shown as Io, IA and IB. The different alleles code for the presence or absence of antigens on the surface of the erythrocytes. 1o codes for no antigens, 1A codes for antigen A and IB codes for antigen B
- Io is recessive. You have to be homozygous to have the blood group O
- IA and IB are codominant. Both alleles are expressed and produce their proteins which act together without mixing. An individual who inheits both IA and IB will have antigen A and antigen B on the surface of their erthrocytes and they will have the blood group AB.
What are digenic crosses?
Breeding experiments involving the inheritance of two pairs of contrasting characteristics at rhe same time
How would you draw a digenic cross?
- state parental phenotypes
- state parental genotypes
- draw gametes
- draw four by four punnet square
- state the offspring phenotypes
What are parental phenotypes?
They describe offspring that have the same phenotypes as the parental organisms
What are recombinant phenotypes?
They describe offspring that have different phenotypes to their parents as a result of recombination of the chromosomes during sexual reproduction
When is it most likely the genetic cross ratios will not be what you expect?
- small sample size
- experimental errors (e.g. when organisms escape or die)
- the process is random and so sometimes the unexpected happens
- unexpected ratios can mean the genes being examined are both the same chromosome (are linked)
When crosses become more complex what an you use to predict the possible offspring and their ratios?
Probabilities
What is the chi squared (x2) test?
A statistical test to compare differences between sets of data to evaluate whether they differ from eachother significantly
What is the formula for the chi squared test and what does each symbol mean?
X2 = sum of (O - E)2/ E
O = observed result E = expected result
What is gene linkage?
When genes for two characteristics are found on the same chromosome and are close together so they are linked and inherited as a single unitq
How can you identify linked genes?
- they will always be found on the same chromosome whereas unlinked genes will be found on seperate chromosomes and approximately equal numbers of gametes containing the parental alleles and recombinant alleles are formed
- when genes are closely linked (when they are close to eachother on the chromosome) the number of recombination events in meiosis will be higher
What is the tightness of the linkage related to?
How close together the linked genes are on the chromosome.
Which linked genes are more likely to be split during the crossing over stage of meiosis?
Genes that are further apart on a chromosome
What is the clue that linkage is involved in a dihybrid cross?
If the expected 9:3:3:1 ratio for a heterozygote cross does not occur and this happens in large numbers
How do you work over a crossover value to build a genetic map of a chromosome.
Crossover value = number of recombinant offspring x 100/ number of total offspring
What is a homogametic individual?
An individual that produces gametes that only contain one type of sex chromosome (e.g. a female has two large X chromsomes)
What is a heterogametic individual?
An individual that produces gametes that contain two different types of sex chromosome (e.g. a male with XY)
Describe the Y chromosome
- has around 23 million base pairs and inly 78 protein coding genes
- carries little but male sex information including the SRY gene which triggers the development of the testes and through the hormones produces by yhe testes the maleness of the fetus
Describe the X chromosome
- much bigger than the Y chromosome with over 150 million base pairs and around 800-1200 protein coding genes
- the X chromosome codes for all the female characteristics and also carries a variety of other genes coding for traits including the clotting factors in the blood and the ability to distinguish between certain colours
Why can even though X and Y are different chromosomes they can still pair up during cell division?
Because they are similar enough
What are genes carried on the X chromosome said to be?
Sex linked
Why will any recessive or mutant alleles passed on the X chromosome from a mother to her sons be expressed in the phenotype?
Because their is no corresponding allele on the homogolous Y chromosome
What is albinism?
A condition seen in many different species in which the natural melanin pigment of the skin, eyes and hair does not form
What is one of the most common forms of albinism due to?
Due to a mutant allele that prevents the formation of a normal enzyme in the cells. The enzyme tyrosinase which is normally active in the melanocytes or pigment forming cells is not formes correctly and so the reactions that make melanin can not take place. It is a recessive phenotype
What is a genetic pedigree diagram?
It shows us what happens in reality in a human family over a long period of time. It includes all the members of a family, indicaring their sex and whether or not they have the disease
What are genetic pedigree diagrams useful for?
- predicitng which family members may be carriers of a genetic mutation e.g. for cystic fibrosis because they highlight carriers of a recessive phenotype
- thry are useful for identifying sex-linked traits becaude a family tree indicates the males and females in a family
Apart from in humans when else are genetic pedigree diagrams often used?
When people selectively breed animals
What is a sex linked disease?
A genetic disease that results from a mutated gene carried on the sex chromosome
What is the ability to see in colour tbe result of?
Multiple genes coding for different aspects of the process of light sensitive cells detecting
Where are many of the genes that code for our ability to see in colour found?
On the X chromosome
What is red-green colour blindness the result of?
A mutation in one of the genes coding for colour on the X chromosome. It is due to a recessive mutation on the gene on the X chromodome
What is Haemophillia?
A sex linked genetic disease in which one of the proteins needed for the clotting of the blood is not made in the body
Why are problems with blood clotting often sex-linked diseases?
Because many of the genes that code for components of the blood clotting cascade are carried on the X chromosome
How are haemophilliacs now treated?
Genetically engineered recombinant bacteria produce pure (clotting) factor VIII in large quantities. Regular treatment with pure factor VIII made by bacteria is used
