Chapter 17 - Dihybrid and sex-linked crosses Flashcards
Define Dihybrid Cross.
A dihybrid cross involves the inheritance of two characteristics.
What is the Law of Independant Assortment?
The Law of independant assortment states that when gametes are formed, either of a pair of alleles is equally likely to combine with either or another pair of alleles.
Is the Law of Independant Assortment Gregor Mendal’s first or second law?
Second law.
What form of cell division causes the law of independant assortment?
Meiosis
What is Sex determined by?
Sex is determined by a pair of sex chromosomes (the X and Y chromosomes ). Females are XX and Males are XY.
What are linked genes
Linked genes are genes that are located on the same chromosome and tend to be transmitted together into the gamete.
How many pairs of chromosomes do humans have?
23 pairs.
What are 22 pairs of human chromosomes known as?
22 pairs of human chromosomes are known as autosomes
One pair of human chromosomes is known as what?
One pair of human chromosomes is known as sex chromosomes.
Name the sex chromosomes
X and Y
What do sex chromosomes determine?
The sex chromosomes determine the sex of the individual.
What 2 chromosomes do females have
Females have two X chromosomes.
What 2 chromosomes do males have
Males have an X and Y chromosome.
Define Sex chromosome
Sex chromosome is a chromosome that determines the sex of an organism
Which chromosome is smaller than the other?
The Y chromosome is much smaller than the X chromosme.
Does the Y chromosome have the same amount of information on it as the X chromosome?
The Y chromosome does not have the same amount of information on it as the X chromosome.
Why are males responsible for sex determination of their offspring?
Males are responsible for sex determination of their offspring as males contain two different chromosomes, X and Y.
When meiosis occurs, females produce gametes that all contain what chromosome?
When meiosis occurs, females produce gametes that all contain the X chromosome.
When meiosis occurs, half of the male’s gametes carry what?
When meiosis occurs, half of the males gametes contain the X chromosome and half carry the Y chromosome.
On fertilisation, the egg cell contributes what?
On fertilisation, the egg cell contributes an X chromosome.
What does the male sperm determine?
The male sperm determines the sex of the offspring.
If an X sperm cell fertilises the egg, what will the offspring be?
If an X sperm cell fertilises the egg, the offspring will be female (XX)
If a Y sperm cell fertilises the egg, what will the offsprig be?
If a Y sperm cell fertilises the egg, the offspring will be male. (XY)
Every time a male and female are mated, what are the chances that the offspring will be male or female?
Every time a male and female are mated, there is a 50% chance that the offspring will be female and a 50% chance that the offspring will be male.
In the dairy industry, why does the 50% probability of male or female have particular consequences?
In the dairy industry, the 50% probability of male or female has particular consequences as female calves are often preferred over male calves.
Scientists involved in cattle breeding have done what for the reason of particular consequences of 50% male or female in dairy industry?
Scientists involved in cattle breeding have determined a way of separating sperm carrying X chromosomes from sperm carrying Y chromosomes in order to produce semen samples that have been sexed.
In mammals, what sex determines the sex of the offspring?
In mammals, the male determines the sex of the offspring.
In birds, what sex determines the sex of the offsprig?
In birds, the female determines the sex of the offspring.
In birds, what are the 2 chromosomes called?
In birds, the 2 chromosomes are called Z and W.
In birds, which chromosome is larger and has more genes?
In birds, the Z chromosome is larger and has more genes than the W chromosome.
What are the 2 sex chromosomes for female birds?
The 2 sex chromosomes for female birds are ZW.
In birds, what are the 2 sex chromosomes for males?
In birds, the 2 sex chromosomes for males are ZZ.
Define Sex Linkage
Sex-Linkage or X-Linkage is a gene that is found on the X cheomosome but there is no copy on the Y chromosome.
Name the fruit fly that Thomas Hunt Morgan conducted genetic crosses with.
Thomas Hunt Morgan conducted genetic crosses with the fruit fly Drosophila melanagaster.
What did Morgan intend to use fruit flies for?
Thomas Hunt Morgan intended to use fruitflies to reproduce Mendel’s monohybrid crosses.
What colour eyes do fruit flies usually have, and what did Thomas Hunt Morgan notice in one of his crosses?
Fruit flies usually have red eyes, however, Thomaa Hubt Morgan noticed a white-eyed male in one of his crosses.
What did Thomas Hunt Morgan do with the white-eyed male fruit fly?
He crossed the white-eyed male fruitfly with a red-eyed female fruitfly and all the F1 produced had red eyes.
What did Morgan naturally presume after this result ?
That red eyes were dominant in fruit flies and white eyes were recessive.
Give 2 examples of sex linkage or x linkage
In fruit flies the gene for eye colour is located on the X chromosome, and in humans the gene for blood clotting and colour vision are carried on the X chromosome.
Name three characteristics caused by recessive genes.
White eye, haemophilia and colour blindness are all caused by recessive genes.
Why do sex-linked disorders occur more commonly in males than in females?
Sex-linked disorders occur more commonly in males than in females because sex-linked genes are carried on the X chromosome and males only have one X chromosome, and therefore only one copy of the gene.
How would a female be affected by a sex-linked characteristic?
In order for a female to be affected by a sex-linked characteristic, she would have to inherit 2 copies of the recessive gene: her mother would have to be a carrier of the sex-linked characteristic and her father would have to have the sex-linked condition.
When is a mendelian ratio of 9:3:3:1 produced in a dihybrid cross
When both parents are heterozygous for both characteristics
Do linked genes follow mendel’s law of independant assortment?
No, linked genes do not follow mendel’s law of independant assortment.
What can separate linked genes?
Crossing over (which occurs during meiosis) can separate linked genes.
Why does the male determine the sex of the offspring?
The male determines the sex of the offspring as 50% of the male gametes carry an X chromosome and 50% carry a Y chromosome. Females produce gametes that carry an X chromosome.
Give an example of an X-linked gene
White eyed fruit flies are an example of an X-linked gene.
What is a very rare sex-linked characteristic in Holstein Cattle? How is it likely to happen?
Male calves can be born with very little hair and no incisors. This sex-linked characteristic is more likely to happen if closely related animals are crossed.
What are the advantages of using fruit flies when studying genetics
These are the advantages of using fruit flies when studying genetics:
- Fruit flies are easy to keep
- They produce a large number of offspring, which is good for statistical analysis.
- They produce a new generation every two weeks.
- They only have four pairs of chromosomes
- They have well documented mutations, e.g. inheritance of white eyes in male fruit flies.
What is a mutation
A Mutation is a permanent change in the DNA sequence of a gene.
What are mutations in genes a source of and what can they lead to?
Mutations in genes are a source of new alleles and can lead to variations.
Give an example of something that was caused by a mutation.
The appearance of the white-eye allele in fruit flies was caused by a mutation in the gene for eye colour.
Are mutations rare and random events?
Yes, mutations are rare and random events.
Give a well-known example of a mutation
A well-known example of a mutation is sickle-cell anaemia.
Describe how sickle cell is formed.
The mutation occurs in the gene for the production of the haemoglobin molecule in red blood cells. This produces red blood cells that are an abnormal, rigid sickle shape.
What can mutations also lead to?
Mutations can also lead to changes in chromosome numbers, and result in the formation of cells that are polyploidy.
What are factors that cause mutations known as?
Factors that cause mutations are known as mutagens.
What do mutagens do?
Mutagens interact with the DNA and cause changes to the DNA sequence.
Name 5 factors that cause mutations.
X-Rays, UV Light, Radiation, radon and colchicine are all factors that cause mutations.
Define Mutagen and give 5 examples of a mutagen.
Mutagen is an agent that can bring about a genetic mutation. Examples are X-Rays, Radon, Colchicine and UV Light.
