dihybrid crosses and patterns of inheritance (pedigrees)

Dihybrid crossesand

Patterns of inheritance (pedigrees)

Lesson 5.

Learning Goals

P Phenotypes

Round Yellow seed

X Wrinkled Green seed

(Pure bred)

Genotypes

RRYY rryy

meiosis meiosis

Gametes RY ry

fertilisation

F1 Phenotypes

RrYy (Selfed)

Genotypes

Round Yellow

Proportions

100%

Dihybrid cross genetic diagram

A dihybrid cross can be treated as two

separate monohybrid crosses

The expected probability of each type of seed can be calculated:

• Probability of an F2 seed being round = 75% or ¾

• Probability of an F2 seed being wrinkled =

• Probability of an F2 seed being yellow =

• Probability of an F2 seed being green =

A dihybrid cross can be treated as two separate monohybrid crosses

The expected probability of each type of seed can be calculated:

• Probability of an F2 seed being round = 75% or ¾

• Probability of an F2 seed being wrinkled = 25% or ¼

• Probability of an F2 seed being yellow = 75% or ¾

• Probability of an F2 seed being green = 25% or ¼

THE LAW OF INDEPENDENT ASSORTMENT

• It appears that the inheritance of seed shape has no influence over the inheritance of seed colour

• The two characters are inherited INDEPENDENTLY

• The pairs of alleles that control these two characters assort themselves independently

Mendel & Meiosis

• The pairs of chromosomes could orientate in different ways at Anaphase 1

Patterns of InheritancePatterns of Inheritance

PedigreesPedigrees

• A pedigree is a genetic family tree that shows how prevalent a trait is in a family unit from generation to generation.

• They are often used to track the expression of genetic conditions and disorders.

PedigreesPedigrees• Squares represent males

and circles females.

• A coloured in shape means that person has the trait in question.

• A half coloured in shape means that they are carrying an allele for a recessive trait.

Anatomy of a Pedigree

Male (left) Female (right)

Affected individuals

Carriers (Heterozygotes for autosomal recessive)

Deceased individuals

Sex unspecified

Autosomal Dominant InheritanceAutosomal Dominant Inheritance• Autosomal means not on the sex chromosomes.• Refers to those situations in which a single copy

of an allele is sufficient to cause expression of a trait.

Autosomal Dominant InheritanceAutosomal Dominant Inheritance• 1. Every affected person should have at least one

affected parent. • 2. Males and females should be equally often affected. • 3. An affected person has at least a 50% chance of

transmitting the dominant allele to each offspring.

Characteristics of a Dominant Pedigree• An affected individual has at least one affected

parent• As a result, dominant traits show a vertical

pattern of inheritance – the trait shows up every generation

• Two affected individuals may have unaffected children

AA Aa

Aa aa

A a

a

a

A

Autosomal Dominant InheritanceAutosomal Dominant Inheritanceexamplesexamples

• Progeria (caused by a mutation) in which the person ages very rapidly. They die before they can reproduce.

• Huntington’s Disease in which the central nervous system starts to break down around the age of 30.

Autosomal Recessive InheritanceAutosomal Recessive Inheritance• Refers to those situations where two recessive

alleles result in a trait being expressed.

Autosomal Recessive InheritanceAutosomal Recessive Inheritance• 1. An affected person may not have affected parents.

Parents would be carriers.• 2. Affects both sexes equally. Can appear to skip

generations.• 3. Two affected parents will have affected children 100%

of the time.

Characteristics of Recessive Pedigrees

• In pedigrees involving rare traits, a horizontal pattern of inheritance is observed

– the trait may not appear in every generation

• An individual who is affected may have parents who are not affected, particularly as a result of consagineous matings

• All the children of two affected individuals are affected

aa aaaa aa

a a

a

a

Autosomal Recessive ExamplesAutosomal Recessive Examples• Albinism is a genetic condition which is the loss of pigment in hair, skin

and eyes.• Tay Sachs is a genetic disorder which is a build up of fatty deposits in

the brain, eventually proving to be fatal.• Cystic Fibrosis is the most common fatal genetic disorder. Mutation in

chloride transport protein that causes thick mucus to build up in lungs

Pedigree of a family with some members showing Huntington disease

Huntington disease is

Huntington disease is

Genetic Tests• Karyotype

• Fluorescence in situ hybridization (FISH)– Details chromosomal abnormalities through

fluorescent tags on chromosomes

• Gene testing– Analyzes specific sequence of gene. i.e. breast

cancer susceptibility gene BRCA1 and BRCA2

• Biochemical testing– Analyzes abnormal enzymes and proteins (Tay

Sachs)

FISH

FISH

Genetic Test for Cystic Fibrosis

• In 1989 researchers at Sick Kids identified the gene for cystic fibrosis

• Gene was on chromosome 7 and named CFTR (cystic fibrosis transmembrane conductance regulator)

• Over 1600 possible mutations in CFTR!

• Genetic tests can identify mutations 85-90% of the time

• 1 in every 3600 children in Canada are born with CF

Genetic test for Huntington disease

• In 1983 the gene for Huntington disease called huntingtin was the first human disease-associated gene to be mapped to a chromosome

• In 1993 huntingtin gene was sequenced– CAG repeats

• Current genetic test looks for CAG repeats (36-39 repeats necessary for disease)

Genetic Counselling

• If there is a family history of a disease, couples may consult a genetic counsellor

• Use pedigrees to determine genotypes of the family members

• Explains probability on passing on disease-causing allele to children

Issues with genetic screening• Why carry out genetic screening at all?• When is a test accurate and comprehensive

enough to be used as the basis for screening?• Once an accurate test becomes available at

reasonable cost, should screening become required or optional?

• If a screening program is established, who should be tested?

• Should private companies and insurance companies have access to employees and client test results?

• What education needs to be provided regarding test results?

Gene Therapy: A future cure?

• Technique aimed at treating genetic disorders by introducing the correct form of the defective gene into a patient’s genome

• Copy of “normal” gene is inserted into a vector (usually viral DNA)

• Virus infects cells and delivers gene into chromosomes

Future of Gene Therapy

• Still in experimental stages due to two obstacles:– immune response to viral vector and poor

integration into target chromosome

• In 2000 St. Michael’s Hospital became performed Canada’s first gene therapy for treatment of heart disease; gene produced protein for stimulation of new blood vessels