Lecture 3 MODE OF INHERITANCE, Study Guides, Projects, Research of Human Genetics

Download Lecture 3 MODE OF INHERITANCE and more Study Guides, Projects, Research Human Genetics in PDF only on Docsity!

Human Genetics

Lecture 3

MODE OF INHERITANCE

Human Genetics Team

Lecture

Objectives

Asses Mendel’s laws of inheritance.

Understand the bases of Mendelian inheritance.

Define various patterns of single gene inheritance

using family pedigree and Punnett’s squares.

Mendel was fortunate he

chose the Garden Pea

(البازالء)

• Mendel probably chose to

work with peas because they

are available in many varieties.

• The use of peas also gave

Mendel strict control over

which plants mated.

• Fortunately, the pea traits are

distinct and were clearly

contrasting. To understand the picture

above: Stamen= العضو الذكري في الزهرة Carpel = الزهرةالعضو األنثوي في Pollen= حبيبات اللقاح Pollinated = ملقحة

*** Mendel’s breeding experiments:**

Interpretation of his results

• The plant characteristics being studied were each controlled by a pair of factors, one of which was inherited from each parent.
• Homozygous : ( Homo = Same ) The pure-bred plants,

with two identical genes.

• Heterozygous : ( Hetero = Different ) The hybrid F

(First generation ) plants, each of which has one gene

for tallness and one for shortness.

- Allelomorphs or alleles : The genes responsible for

these contrasting characteristics.

* Genotypes and Phenotypes

- Homozygous dominant TT

• Homozygous recessive tt
• Heterozyous Tt

Two alleles forming one gene

Two alleles form one gene ( each gene has two alleles coming from each parent )

The dominant gene can have homozygous or heterozygous alleles BUT: the recessive gene ALWAYS have homozygous alleles and can NEVER be heterozygous

* First Experiment :

* Second Experiment :

P1 generation ( Partenal generation)

F1 generation

( Filial generation )

100% Purple

F1 generation

(First generation)

F 2 generation (second generation) 75% Purple and 25% White

* Law of Dominance:

In the monohybrid cross (mating of two organisms that differ in only one character), one version disappeared.

• Genotype versus phenotype :

) Pp ( غير نقية% أزهار بنفسجية 011 في الجيل األصلي ( األبوين ) يكون الناتج: سائدة) ينتج ( Pp ( غير نقيةمع زهره اخرى بنفسجيه ( Pp ( غير نقية وعند تزاوج الجيل االول زهره بنفسجيه 3:0ازهار بنفسجيه وواحده متنحيه بيضاء 3 لدينا

3 purple flowers: Only one is homozygous PP and 2 are heterozygous Pp

1 white flower: ALWAYS homozygous pp

Mendel’s 3 rd^ Law of Inheritance :

Phenotypic ratio: 9 round, green: 3 round, yellow: 3

wrinkled, green: 1 wrinkled, yellow  (9: 3:3:1)

Genotypic ratio:

1 RRGG: 2 RRGg: 2 RrGG: 4 RrGg: 1 RRgg: 2 Rrgg: 2 rrGg: 1 rrGG: 1 rrgg

2 pair mix cross the system will be 9:3:3:

What is the difference between mendel’s first law third law? First law: he studied the characteristics of only ONE trait for example : the color of the flowers Third law: he studied TWO traits for example: the color of flowers + shape of the seed

* THE LAW OF INDEPENDENT ASSORTMENT :

It refers to the fact that members of different gene pairs

segregate to offspring independently of one another.

In reality, this is not always true , as genes that are close together on the same chromosome tend to be inherited together, i.e. they are "linked"

• MENDELIAN INHERITANCE (simple pattern of inheritance)

• Over 16,000 traits/disorders in humans exhibit single gene unifactorial or Mendelian inheritance.
• A trait or disorder that is determined by a gene on an autosome is said to show autosomal inheritance.
• A trait or disorder determined by a gene on one of the sex chromosomes is said to show sex-linked inheritance.

MODES OF INHERITANCE OF SINGLE GENE DISORDERS

Autosomal Sex Linked

Recessive Dominant Y Linked X Linked

Recessive Dominant

* Examples of Autosomal dominant disorders:

• Familial hypercholesterolemia (LDLR deficiency) - Adult polycystic kidney disease - Huntington disease

Myotonic dystrophy

• Neurofibromatosis type 1
• Marfan syndrome

*** Autosomal Recessive Inheritance :**

• The trait (character, disease) is recessive
• The trait expresses itself only in homozygous state
• Unaffected persons (heterozygotes) may have affected children (if the other parent is heterozygote

(األقارب) - The parents of the affected child maybe related. (consanguineous )

- Males and female are equally affected.

* Examples of Autosomal Recessive

Disorders:

- Cystic fibrosis - -Thalassaemia

• Phenyketonuria - Recessive blindness
• Sickle cell anaemia - Mucopolysaccharidosis

*** Sex – Linked Inheritance :**

- This is the inheritance of a gene present on the sex chromosomes.

• The Inheritance Pattern is different from the autosomal inheritance.
  • Inheritance is different in the males and females. - The gene is on the Y chromosomes.
• The gene is passed from fathers to sons only. - Daughters are not affected. - Hairy ears in India.
• Male are Hemizygous, the condition exhibits itself whether dominant or recessive.

Y – Linked Inheritance

• The incidence of the X-linked disease is higher in male than in female. ( because they are Hemizyous)
• The trait is passed from an affected man through all his daughters to half their sons.
• The trait is never transmitted directly from father to sons. (because sons only get “ Y ” chromosome from their father)
• An affected women has affected sons and carrier daughters. (since they are affected they have only recessive alleles let ’ s say for example “ rr ” , so their sons will definitely get an r which will affect the sons, on the other hand daughter will get also an “ r ” but they are not hemizygous so depending on the other allele that they will get from their fathers they either become Carrier “ Rr ” or if the father is affected too they will become affected also “ rr ” .)

(1) Normal female, affected male : - All sons are normal All daughters carriers “ not affected **”

2. Carrier female, normalmale: -** 50% sons affected 50% daughters carriers

(3) Homozygous female, normal male:

- All daughters carriers.

• All sons affected.

X – Linked Recessive Inheritance

- Albinism (Ocular) - Fragile X syndrome - Hemophilia A and B

• Lesch – Nyhan syndrome - Mucopoly Saccharidosis 11 " Hunter ’ s syndrome "
  • Muscular dystrophy (Duchenne and Beeker ’ s)
• G- 6 - PD deficiency - Retinitis pigmentosa - Incontinentia pigmenti (IP) Lethal in males during the prenatal period Lethal in hemizygous males before birth: Exclusive in females (because its lethal in males before birth)

Affected female produces : affected daughters normal daughters normal sons

X - Linked Recessive Disorders

X - Linked dominant Disorders

in equal proportions (1:1:1)