Topic 16

Question 1

Dihybrid crosses

Answer 1

A dihybrid cross involves observing the inheritance of two characteristics controlled by two different genes.

Question 2

Unlinked genes

Answer 2

The two genes involved are located on non homologous chromosomes and so the genes are inherited independently.

Question 3

How do gametes get unlinked genes?

Answer 3

After meiosis each gamete receives one chromosome of each pair due to disjunction (segregation) of homologous chromosome pairs during anaphase I of meiosis.
So one allele of each gene is present in the gamete as only one member of each homologous pair is present in the gamete.
Each chromosome pair asserts independently of other chromosome pairs so this results in independent assortment of the two genes during meiosis

Question 4

Explain: each chromosome pair assorts independently of other chromosome pairs

Answer 4

The movements of the maternal and paternal chromosomes of one pair to particular poles does not influence the movements of other maternal and paternal chromosomes to a particular pole.

Question 5

Parents:P

Answer 5

Only used for a cross between true breeding (homozygous) parents

Question 6

Selfed

Answer 6

The F1 are mated with each other

Question 7

F2

Answer 7

The heterozygous offspring of a cross between two true breeding parents.

Question 8

F2

Answer 8

The offspring produced by crossing f1 heterozygotes

Question 9

Dihybrid test cross (unlinked)

Answer 9

Shows whether two genes are found on different chromosomes or whether they are found on the same chromosome

Question 10

Dihybrid cross (unlinked)

Answer 10

A cross involving the inheritance of 2 characters determined by 2 genes located on different chromosome pairs

Question 11

Number of types of gametes produced in a heterozygote

Answer 11

4

Question 12

Number of types of gametes produced in a homozygote

Answer 12

One

Question 13

Linked genes

Answer 13

Genes that are found on the same chromosome.
Genes on the same chromosome are said to be linked and they are almost always inherited together as a unit and do not undergo crossing over.

Question 14

Linkage

Answer 14

Linkage reduces the variation of offspring that can be produced.
This alters the expected genotype and phenotype ratios in the offspring

Question 15

Ratio when two heterozygotes are crossed at both gene loci

Answer 15

9:3:3:1

Question 16

Ratio when a heterozygote is crossed with a homozygous recessive individual at both gene loci (test cross)

Answer 16

1:1:1:1

Question 17

Linked genes broken

Answer 17

Linked genes can be broken by crossing over during meiosis. Crossing over and recombination is the physical exchange of genetic material between two homologous chromosomes, which occurs during prophase I of meiosis.

Question 18

Crossing over in linked genes

Answer 18

The effect of cross over is to provide a further source of variation in the expected offspring genotype ratios.
A cross over point is more likely to occur between two genes that are widely separated on a chromosome than between two gene loci that are close together.
The closer the genes are on the chromosome, the lower the chance of crossover.

Question 19

Linked or not linked?

Answer 19

It is possible to predict whether two genes are linked or not from results with a test cross.
No linkage- equal amounts of the four possible genotypes are produced.
Linkage- more offspring with parental genotypes are produced than offspring with recombinant genotypes

Question 20

Describe the effect of linkage on the inheritance of genes

Answer 20

Linkage reduces the variety of offspring that can be produced. Linked genes tend to be inherited together because they are so close together and fewer genetic combinations of their alleles are possible.

Question 21

Explain how linkage decreases the amount of genetic variation in offspring

Answer 21

The closer together they are, the more likely to be inherited together and so fewer genetic combinations are produced.

Question 22

Genetic recombination

Answer 22

Refers to the exchange of alleles between homologous chromosomes as a result of crossing over.

Question 23

Pedigree charts

Answer 23

Enable the study of patterns of inheritance in humans for a particular trait

Question 24

Autosomal dominant

Answer 24

Either sex can be affected
All affected individuals have atleast one affected parent
Once the trait disappears from the branch it does not reappear (unless a new mutation occurs)
Unaffected parents cannot produce affected offspring (unless a mutation occurs)

Question 25

Autosomal recessive

Answer 25

Either sex affected Unaffected parents can produce affected offspring The trait can disappear and then reappear

Question 26

X linked dominant

Answer 26

More females affected than males Affected males pass on the trait to all daughters and no sons Affected male always has affected mother

Question 27

X linked recessive

Answer 27

More common in males than females Affected females pass the trait to all sons No sons of affected male can inherit the disease Unaffected parents can produce only affected male offspring

Question 28

Y linked genes

Answer 28

Transmitted exclusively from males to sons only Affected Male cannot pass the trait to daughters No affected females in pedigree Cannot skip a generation and then reappear

Question 29

MtDNA

Answer 29

Mother passes trait to all offspring | But only females can pass it on

Question 30

Genetic screening

Answer 30

The testing of segments of a population as part of an organised program, for the purpose of detecting inherited disorders

Question 31

How is genetic screening of babies carried out

Answer 31

In the first few days after birth a baby's heel is pricked and a few drops of blood are placed on a card made of special absorbent paper. Sample is sent to a lab where tests for the genetic disorders are carried out on the dried blood Testing for other rare metabolic disorders may also occur

Question 32

Genetic testing

Answer 32

Scientific testing of an individual's genotype | May be carried out on people who are at a high risk of inheriting a faulty gene based on family history

Question 33

Other reasons for genetic testing include

Answer 33

Identify paternity Diagnose a condition in someone showing symptoms Guide medical treatment of a person Identify the carrier status of a person for an inherited condition Forensic purposes

Question 34

Positive implications of genetic testing

Answer 34

Early detection prevents serious negative consequences on mental and physical development Gives certainty in an otherwise uncertain situation Allaying fears when testing shows that a person has not inherited a particular genetic defect

Question 35

Negative implications of genetic testing

Answer 35

Some people may not be able to cope with finding that they have inherited a faulty allele with serious clinical consequences A test result for one person may have implications for many family members