Inheritance and Pedigrees

Inheritance

Inheritancethe passing on of characteristics from one generation to the next.

Geneticsthe study of inheritance

Phenotypecharacteristics or features

Genotypedetermined by genetic information in the chromosomes that you received in the gametes of your parents

Phenotype

Phenotype= genotype + environment

Environmental factors

Environmental factors contribute to characteristics that make up your phenotype.

E.g. your weight is determined by genetic factors, but is also influenced by what you eat and how active you are.

Dominant and Recessive

Dominant traitsthe trait that appeared in the F1

generation

Recessive traits the trait that was 'masked' (hidden) in the F1 generation and reappeared in the F2 generation

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The two forms of the same gene are called alleles.

If the allele for a dominant trait is present, it will always be expressed. The recessive trait is hidden.

Punnet Squares

A Punnett square is a diagram that is used to predict the outcome of a genetic cross.

It shows which alleles for a particular trait are present in the gametes of each parent.

It then shows the possible ways in which these can be combined.

Punnet rules

When using a Punnett square for dominant/recessive inheritance, you use a capital letter for the allele of the dominant trait (B) and a lower case letter for the allele of the recessive trait (b)

Other types of inheritance

Simple inheritance can't explain all inheritance sometimes alleles blend in some way.

Codominance

Co-dominant - a heterozygote has a phenotype where two different alleles are expressed they show both traits.

Eg. A flower has red & white parts (have the red allele & the white allele).

Incomplete dominance

Incomplete dominance - a heterozygote has a phenotype halfway between homozygotes.

Eg. A pink flower has the red & white allele but the phenotype is a blend of both.

Complex Inheritance

Not every characteristic is controlled by a single gene.

Many characteristics are controlled by a number of genes this results in

more variation.

E

E.g. Human height & skin colour

What is a pedigree

Pedigree - pictorial family tree where individuals who show a particular disease or characteristic are marked.

Pedigrees can be used to find patterns of inheritance.

Different symbols are used to draw pedigrees.

Autosomal recessive

Parent : Both must be heterozygous or homozygous recessive

GENERATION Trait may skip generations

OFFSPRING Males and females are affected equally

- unaffected parents can have affected children

Autosomal Dominant

PARENT: One parent is affected

GENERATION: trait does not skip generations

OFFSPRING: only half the children are affected -males and females affected equally

- when father transfers disease to his son -- it is always an autosomal trait.

- affected parents can have unaffected children.

X linked recessive

PARENT : no father to son transmission only mother to son

GENERATION: trait skips generation

OFFSPRING: males are mostly affected

Could also transfer from father to daughter

Pedigree rules for parents

A male and female directly connected by a horizontal line have mated and have children.

• Vertical lines connect parents to their children.

Autosomes

The 22 chromosome pairs other than the XX (female) or XY (chromosomes)

Allele - A version of a gene.

Pedigree Key Hints

(1) An unaffected individual can be a carrier (have one allele) of a recessive trait. (because two alleles of a recessive trait are required for an individual to be affected)

(2) When a trait is X-linked, a single recessive allele is sufficient for a male to be affected. (because the male is hemizygous - he only has one allele of an X-linked trait)

(3) A father transmits his allele of X-linked genes to his daughters, but not his sons. A mother transmits an allele of X-linked genes to both her daughters and her sons.

Patterns that Indicate a RECESSIVE Trait

The disease must be RECESSIVE if any affected individual has 2 unaffected parents.

Since this is a genetic disease at least one parent must have an allele for the disease.

I

lf neither parent is affected, the trait cannot be dominant.

Autosomal recessive

If any affected founding daughter has 2 unaffected parents the disease must be autosomal recessive.

An affected individual must inherit a recessive allele from both parents, so both parents must have an allele.

I

f the father had a recessive X-linked allele, he would have to be affected (since he only has one X-linked allele).

Patterns that indicate a DOMINANT trait

The disease must be DOMINANT if every affected child of NON-FOUNDING parents has an affected parent.

The unaffected mother, who is marrying in, does not carry an allele for the disease;

so the affected child inherits an allele only from the affected father.

No child could be affected by a single autosomal recessive allele, or X-linked recessive allele, so the trait is dominant.

Autosomal dominant

When an affected son of non-founding parents has an affected father the disease must be AUTOSOMAL DOMINANT.

A father does not transmit X-linked alleles to a son, so the disease cannot be X-linked dominant.

•

When an affected daughter of non-founding parents has an affected father, we cannot determine whether the DOMINANT disease is autosomal or x-linked.

The affected father can transmit either an autosomal dominant allele, or an X-linked dominant allele to his daughter.

Trait

is the term used to describe a particular characteristic of an organism.

Karyotype

A picture created by staining, arranging and numbering chromosomes in pairs by size, centromere position and banding pattern.

Chromosome

4. A thread-like linear strand of DNA and associated proteins that carries genes and is responsible for the transmission of hereditary information

Chromatid

is one of the two identical copies of DNA making up a duplicated chromosome, which are joined together at their centromeres for the process of cell division.

Centromere

The region on a chromosome that joins two sister chromatids

Gene

The region of DNA containing instructions for making a protein is known as a gene.

Alleles

Different forms of the same gene are known as alleles.

Genotype

genetic makeup of an organism, written as a code Aa or BB for example

carrier

A carrier does not show the expression of one copy of a recessive gene. They may pass a disease onto their offspring but do not have the disease themselves.

Replication, , diploid

DNA replication is the process by which genetic material is copied in order for cell division to occur.

25. When cells of a species contain pairs of chromosomes they are said to be diploid cells.

haploid, independent assortment

27. When cells of a species have one copy of each chromosome they are said to be haploid.

28. The phrase independent assortment refers to the fact that during meiosis different combinations of chromosomes may end up together in the gametes

sex-linked traits

Conditions that are caused by genes on the X chromosome. They are denoted by a superscript letter on an X (e.g. XH).

Mutation

30. A change in the DNA sequence of an organism.

codon

A series of three bases that acts as an instruction to add a particular amino acid to a growing polypeptide chain is a codon