Genes and patterns of inheritance

What is a genotype?

The 2 alleles an organism contains to represent a particular trait

What is a phenotype?

The characteristics of a organism determined by its genes and its environment

What does homozygous mean?

Both alleles of a gene are the same

What does heterozygous mean?

Alleles of the same gene are different

What is a gene?

Length of DNA with a base sequence thst codes for a particular polypeptide

What does homologous mean?

Two chromosomes of a pair in a diploid organism

Where are genes located?

In chromosomes in the nucleus of the cell

What are alleles?

Alternative forms of the same gene

Where are alleles located?

Same locus on homologous chromosomes in diploid organism

How can you tell the difference between alleles?

Each has a slightly different nucleotide base sequence

What is a locus?

The precise location of a gene on a chromosome

What does a monohybrid cross involve?

Inheritance of one characteristic controlled by one gene which may have 2 or more alleles

What is monohybrid inheritance?

The inheritance of the alleles of a single gene

What is Mendel's Law of Segregation?

The characteristics of an organism are determined by internal factors (alleles) which occur in pairs. The 2 alleles of each gene are separated during meiosis and only one enters each gamete.

How can Mendel's law of segregation be explained?

By meiosis

Alleles are located on homologous chromosomes that are seperated during anaphase 1 and gametes only carry one allele

What is a dominant allele?

An allele that is fully expressed in the phenotype of a heterozygote.

What is a recessive allele?

A form of a gene that is not expressed when paired with a dominant allele

What is codominance?

When both alleles contribute to the phenotype

What are lethal alleles?

A lethal allele's phenotype, when expressed, causes the death of an organism. Lethal alleles can be embryonic or postnatal.

Postnatal lethal alleles cause abnormalities in the progeny that cause them to die early on in development. Example: Cystic Fibrosis and Huntington disease

Multiple alleles

One characteristic is controlled by a single gene which has more than 2 alleles

What is sex linkage?

The gene and its alleles are located on a sex chromosome (mostly the X)

What is a test cross?

Crossing an organism with a dominant phenotype with one that has a recessive phenotype

What is dihybrid inheritance?

Inheritance of 2 characteristics controlled by 2 genes on separate chromosomes with 2 or more alleles

Mendel's second law of independent assortment?

During the formation of gametes the segregation of the alleles of one gene is independent to the segregation of alleles of any other gene

How can you show Mendel’s second law?

Meiosis

Homologous pairs align independently of each other on the equator during metaphase 1

The member of one pair segregates with either member of another pair

What do legal allelic combinations cause?

Death of zygote or early embryonic stage

Death after reduced lifespan- tay Sachs disease

Death at early development stage with presence in heterozygote displaying a distinctive phenotype

What is Y-linked inheritance?

Inheritance of genes on the Y chromosome, can only be transmitted from father to son.

What is X-linked inheritance?

A gene passed only through the X chromosomes. Hence fathers cannot pass to sons, etc

X-linked recessive traits

More common in males than females

Has to appear in both parents to affect female

Affected males inherit the allele from female parent since Y chromosome is inherited from the male

X-linked dominant traits

Rarely skip generations

Are indicated in a pedigree where an affected male has all affected daughters but no affected sons

Hypophosphatemia is an example

Autosomal recessive disorders

Tay-Sachs, Cystic fibrosis, sickle cell anemia, phenylketonuria

Autosomal recessive

May skip generations

Parents of affected individual not affected by condition but are carriers of the allele. These disorders require two copies of the mutant allele for phenotypic expression.

X-linked recessive for autosomal recessive

More common in males than females

If female affected father must be affected

Affected female passes trait to sons

Autosomal dominant

A pattern of inheritance where only one copy of a dominant allele is required for phenotypic expression. This means an affected individual has a 50% chance of passing the trait to offspring.