inheritance

lecture 14

What is a hereditary determinant of a trait called?

A gene

What are the different forms of a particular gene called?

Alleles. These can arise from a mutation or alteration in a gene

What confers the phenotype of a 'trait'?

The underlying genotype

What is a dominant trait?

A trait that is expressed in the phenotype when the genotype is either heterozygous or homozygous1.

What are single gene disorders (or monogenic diseases)?

Human diseases caused by a mutation in a single gene

What is noted about the frequency and cause of most single gene disorders?

Most are caused by recessive genes (thousands exist) and are rare

Provide examples of single gene disorders that are non-lethal and life-threatening mentioned in the source.

Non-lethal such as albinism or life-threatening such as Cystic Fibrosis

How can the mode of inheritance (dominant or recessive) for a trait in humans be determined?

By examining the appearance of phenotypes in related individuals across generations

How are recessive genes expressed phenotypically?

Only in homozygotes (when two recessive alleles are inherited from both parents)

In the context of recessive diseases, what role do heterozygotes play?

Heterozygotes can be carriers of a disease allele but are not affected with the disease

In contrast to recessive alleles, how are dominant alleles expressed phenotypically?

In both homozygotes and heterozygotes5

Why can a disease-causing allele lead to health problems?

Because genes code for proteins, a disease-causing allele can code for a malfunctional protein or none at all

What are traits caused by a combination of a genetic component and an environmental component called? Provide examples.

complex or multifactorial traits

Examples given include heart disease, diabetes, and cancer

Describe the inheritance pattern for a trait following Recessive Inheritance.

Both parents carry a normal gene (N) and a faulty, recessive gene (n). The parents are unaffected carriers (Nn). Their offspring can be affected (nn), not affected (NN), or carriers (Nn

Describe the inheritance pattern for a trait following Dominant Inheritance.

One parent has a single, faulty dominant gene (D) that overpowers its normal counterpart (d), affecting that parent (Dd). When mating with an unaffected, non-carrier mate (dd), the offspring are either affected (Dd) or not (dd)

What tool is used to observe the results of segregation (Mendel's 1st law) in human families?

Human pedigrees

What is recorded from family history in a pedigree analysis, and what is then determined?

The Phenotype is recorded from family history, so the genotype can be determined

What are key characteristics of Dominant Inheritance patterns observed in pedigrees?

◦The trait affects both sexes.

◦Every affected person has an affected parent.

◦Approximately half of offspring are affected

What are key characteristics of Recessive Inheritance patterns observed in pedigrees?

◦

The trait affects both sexes.

◦

Most affected persons have parents who are not themselves affected but are heterozygous for the recessive allele (carriers)

◦

The parents of affected individuals are often unaffected carriers

How are dominant and recessive inheritance patterns further defined?

Into those caused by genes on autosomes or sex chromosomes (either X or Y)

How many patterns of Mendelian inheritance are mentioned as being defined by autosomal or sex chromosome location?

5 patterns

What are lethal genes?

Genes which result in the premature death of the organism

Which genotypes can be killed by dominant lethal genes?

heterozygotes or homozygotes

Which genotypes can be killed by recessive lethal genes?

homozygotes

If a lethal allele is passed down and causes death, what is the pattern of inheritance referred to?

Dominant lethal inheritance pattern or recessive lethal inheritance pattern, respectively

What are examples of late acting (presented later in life) lethal genes mentioned?

Huntington's disease and Tay-Sachs disease

Why does a cross between two heterozygous individuals for a trait with a dominant lethal allele result in a 2:1 ratio instead of 3:1?

Because the homozygous dominant condition (e.g., TT) will not survive

State Mendel's 2nd Law of Independent Assortment.

The alleles for two (or more) genes on separate chromosomes segregate from each other independently in meiosis"

What does the Law of Independent Assortment imply about allele combinations in gametes?

All combinations are possible

What does Mendel's 2nd Law look at the inheritance of?

The inheritance of two traits at the same time

How is a monohybrid cross defined?

A cross involving two parents differing in a single character or trait

How is a dihybrid cross defined?

A cross involving two parents differing in two traits

What is the purpose of a Back cross or Test cross?

reveal the genotype of an organism that exhibits a dominant trait

Why did Mendel use the test cross?

o ensure he had true breeding varieties of pea plants

How is a test cross performed for a dihybrid trait

ross the organism with the unknown genotype exhibiting the dominant phenotype with a double recessive genotype (ttpp)