SEX LINKED

RECESSIVE

INHERITENCE

Name : Sheryl Bhatnagar

Roll No. : 2047

Botany(h) 2nd Year

PEDIGREE ANALYSIS

• The term 'pedigree' refers to the record of an organism's ancestry.

• Pedigree charts are diagrams which are constructed to show biological relationships - how a trait can pass from generation to generation.

• From a pedigree, you can determine if a trait is dominant or recessive, if it is sex-linked, and probability of it affecting future generations.

• To build such a chart to see the flow of genotypes, symbols and rules must be followed.

Inheritence Pattern

• The four most common patterns of inheritance of genetic conditions due to a change in a single gene in families are therefore described as:

• Autosomal recessive

• Autosomal dominant

• X-linked recessive

• X-linked dominant

Important points

• Women have two X chromosomes; men have an X and a Y • Genes located on the X chromosome are called X-linked genes. There are

very few genes located on the Y chromosome. • Some variations in genes stop the gene from working properly: the gene is

said to be faulty (mutated). The gene variation can be either ‘dominant’ or ‘recessive’

• A woman who has a ‘recessive’ gene variation in one of her X-linked gene copies and the other copy is working as it should, is a carrier of the recessive faulty gene. She will generally not be affected by the condition.

• Males have no ‘back-up’ working copy and so will generally be affected by the condition if they have the X-linked faulty gene.

• A woman who has a ‘dominant’ change in one of her X-linked gene copies and the other copy is working as it should, is a carrier of the dominant faulty gene. She will generally be affected by the condition.

The inheritance of X-linked recessive

faulty genes

• Men who have the faulty gene copy on their X chromosome do not have a partner chromosome with a working copy of the gene. So they will not be able to send the right information to the cells to make the gene product. Men will be affected by the condition due to the X-linked recessive faulty gene being expressed in the cells, even when the gene variation (mutation) is recessive.

• If the body can still work normally with the available gene product, a woman will generally have no health problems as a result of carrying the X-linked faulty gene copy. The variation making the gene copy faulty is thus hidden or ‘recessive’ to the usual information in the working copy of the gene.

Sex linked inheritence

• The chance that a child will inherit an X-linked recessive condition in every pregnancy is different for sons and daughters and depends on whether the mother or father has the faulty gene:

- When the mother is a carrier of an X-linked recessive faulty gene there is 1 chance in 2 (50% chance) that a son will be affected by the condition and a 1 chance in 2 that a daughter will be a usually unaffected genetic carrier like the mother

- When the father is affected by a condition due to an X-linked recessive faulty gene, none of his sons will be affected but all of his daughters will be carriers of the X-linked recessive faulty gene, although they will generally be unaffected by the condition

Interpreting a Pedigree Chart

• Determine whether the Trait (disorder) is dominant or recessive.

– If the disorder is dominant, one of the parents must have the disorder.

– If the disorder is recessive, neither parent has to have the disorder because they can be heterozygous.

For Sex Linked Traits

– Dominant

• Trend: does not skip generations

• Female: BB or Bb / Male: B

• Any males must have mothers with trait

– Recessive

• Trend: skips generations

• Female: bb / Male: b

• Most affected individuals will be male

• Females with trait must have father with trait

Rules for inheritence

X-Linked Recessive

• More males than females are affected• Affected sons are usually born to unaffected • mothers, thus the trait skips generations• Approximately 1/2 of carrier mothers’ sons are • affected• It is never passed from father to son• All daughters of affected fathers are carriers

Example

• Only males are affected and sons do not share the trait.

• Daughters are carrier.

• phenotype of their father - Thus X-linked.

• skips generations.

• RECESSIVE

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Colorblindness

• Most types are caused by a gene located on the X chromosome

• Red green colorblindness is the most common type

– Amongst Caucasians;

• 8% of males are colorblind

• 1% of females are colorblind

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Test For Colorblindness

• Ishihara Test

• Normal color vision see an “8”

• Colorblind people don’t see anything

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Sex-linked Genetics

• We use an “X” to identify that this characteristic is carried on the X chromosome and a superscript “C” for the dominant (normal) trait or lowercase “c” for the colorblind trait.

• XC = normal vision allele

• Xc = colorblind allele

• Y = normal male chromosome (no allele for colorblindness)

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Hemophilia

• Aka “bleeder’s disease”.

• Much rarer than colorblindness

– 1 in 10,000 males

– 1 in 100,000,000 females

• Hemophilia A is a disorder where the blood cannot clot properly due to a deficiency of a clotting factor called Factor VIII. This results in abnormally heavy bleeding that will not stop, even from a small cut.

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Queen Victoria

• Acquired hemophilia through a mutation– Parents didn’t have it or

pass it on

• Passed it to 1 son and at least two daughters

• The daughters were married to nobility in France and Germany and it caused the loss of at least one heir to the throne!

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What can be done if a woman is a carrier of an X-linked recessive faulty gene?

• If a woman is a genetic carrier of an X-linked recessive condition, she can find out information about the condition about her risk for having an affected child and discuss her reproductive options with a genetic counselor.

• Testing in pregnancy to determine the presence of the faulty gene may be possible.

• Testing of the embryo in association with assisted reproductive technologies (ART) such as pre implantation genetic diagnosis (PGD) may also be possible .

• A discussion with a genetic counselor will assist in enabling a couple to make an informed decision with the most up-to-date information.