Genptype, Phenotype And Inheritance

Question 1

How many genes does each human have and how many copies of each?

Answer 1

• Each human has 25,000 genes
• Each individual has two copies of each given gene (one maternal and one paternal)
• Each individual has two alleles of a gene
• BUT there are many alleles of a gene within a population!

Question 2

Define homozygous

Answer 2

Two alleles of a gene are the same

Question 3

Define heterozygous

Answer 3

Two alleles of a gene are different

Question 4

Define hemizygous

Answer 4

Only one allele o a gene on the X chromosome (i.e males only)

Question 5

Define dominant

Answer 5

The allele in a heterozygous which determines the phenotype

Question 6

Define recessive

Answer 6

The non-dominant allele in a heterozygote, i.e. The allele which does not determine the phenotype in a heterozygote

Question 7

Define co-dominance

Answer 7

Where neither of 2 alleles are dominant over each other e.g. Allele A and B for blood type

Question 8

Describe blood type A

Answer 8

Antigens on membrane: A
Antibodies in blood: anti B
Safe to transfuse to: A, AB
Safe to transfuse from: A, O
Genotype: IAIA or IAIO

Question 9

Describe blood type B

Answer 9

Antigens on membrane: B
Antibodies in blood: anti A
Safe to transfuse to: B, AB
Safe to transfuse from: B, O
Genotype: IBIB or IBIO

Question 10

Describe blood type AB

Answer 10

Antigens on membrane: A + B
Antibodies in blood: none
Safe to transfuse to: AB
Safe to transfuse from: A, B, AB, O
Genotype: IAIB

Question 11

Describe blood type O

Answer 11

Antigens on membrane: none
Antibodies in blood: anti A anti B
Safe to transfuse to: A, B, AB, O
Safe to transfuse from: O
Genotype: IOIO

Question 12

Describe pedigree symbols

Answer 12

■ affected male
□ male
▌carrier male
● affected female
○ female
◐ carrier female
◆ affected unknown sex
◇ unknown sex
Diagonal line from bottom left to top right - decreased
Shaded dot inside symbol - usually x-linked recessive carrier

Question 13

name some inheritance patterns

Answer 13

Autosomal
Sex linked
Mitochondrial
Polygenic

Autosomal recessive
Autosomal dominant

X-linked recessive
X-linked dominant
Y-linked (rare)

Question 14

Describe autosomal recessive inheritance and give an example of an autosomal dominant disease

Answer 14

• Heterozygotes unaffected
• Males and females equally affected
• Two heterozygotes have 25% chance of having affected offspring
• Two affected (homozygous) individuals will have affected offspring only
• Males and females equally affected
• Disease seem to ‘come out of nowhere’
• Disease can skip generations
• Both parents of affected individual are heterozygous ‘carriers’
• For example: Cystic Fibrosis

Question 15

Describe autosomal dominant inheritance and give an example of an autosomal dominant disease

Answer 15

• Heterozygotes affected
• Males and females equally affected
• Disease rarely found in homozygous state (assume heterozygous unless told otherwise)
• Every affected individual has 50% chance of having affected offspring
• Every affected individual will have (at least) one affected parent
• Disease cannot skip a generation, i.e. affected individual(s) in every generation
• For example: Huntington’s Disease

Question 16

Describe X linked recessive inheritance and give an example of an X-linked recessive disease

Answer 16

• Hemizygous males (if their X is affected they are diseased) and homozygous females affected
• Disease more common in males
• Heterozygous female ‘carrier’ has 50% chance of having affected sons
• Affected males cannot give trait to sons (as they only give Y chromosome)
• Males and females unequally affected
• Every affected male will have (at least) a heterozygous ‘carrier’ mother
• Every affected female will have an affected father and a ‘carrier’ mother
• Heterozygous daughters of affected males
• For example: Haemophilia A

Question 17

Describe X-linked dominant inheritance and give an example of an X-linked dominant disease

Answer 17

• Hemizygous males and heterozygous females affected
• Heterozygous ‘affected’ female has 50% chance of having offspring
• Affected males cannot give trait to sons, but will give it to all their daughters (X is affected - will give it to all daughters)
• For example: Rett syndrome

Question 18

Describe mitochondrial inheritace

Answer 18

All individuals inherit from their mother since mtDNA is inherited only from the mother

Question 19

Is inheritance monogenic or polygenic?

Answer 19

Can be polygenic, e.g. Albinism - inherited in a recessive manner
Two genes A1 and A2
2x2 alleles: A1, a1, A2, a2

Therefore 2 albino individuals’ offspring can be normal, e.g. a1a1A2A2 x A1A1a2a2 = a1A1A2a2 (offspring is heterozygous for both)

Question 20

Define linked genes

Answer 20

• Genes on the same chromosome are said to be ‘linked’
• Genes on different chromosomes are said to be ‘not linked’
• Linked genes do not show independent assortment at meiosis
• Recombination frequency between two linked genes is dependent on the distance between the genes
• Genes close together are ‘tightly linked’ - inherited together more frequently
• Genes far apart on the same chromosome almost behave as unlinked genes