3.4: Inheritance

UnderstandingsMendel discovered the principles of inheritance with

experiments in which large numbers of pea plants were crossed.

Gametes are haploid so contain only one allele of each gene.

The two alleles of each gene separate into different haploid daughter nuclei during meiosis.

Fusion of gametes results in diploid zygotes with two alleles of each gene that may be the same allele or different alleles.

Dominant alleles mask the effects of recessive alleles, but codominant alleles have joint effects.

Many genetic diseases in humans are due to recessive alleles of autosomal genes although some genetic diseases are due to dominant or codominant alleles.

Some genetic diseases are sex linked. The pattern of inheritance is different with sex-linked genes due to their location on the sex chromosome.

Many genetic diseases have been identified in humans but most are very rare.

Radiation and mutagenic chemicals increase the mutation rate and can cause genetic diseases and cancer.

Applications/skills

Guidance

A: Inheritance of ABO blood groups. A: Red-green color blindness and hemophilia as examples of sex-linked inheritance. A: Inheritance of cystic fibrosis and Huntington’s disease. A: Consequences of radiation after nuclear bombing of Hiroshima and accident at

Chernobyl S: Construction of Punnett grids for predicting the outcomes of monohybrid genetic

crosses. S: Comparison of predicted and actual outcomes of genetic crosses using real data. S: Analysis of pedigree charts to deduce the pattern of inheritance of genetic diseases.

Alleles carried on X chromosomes should be shown as superscript letters on an uppercase X, such as ___.

The expected notation for ABO blood group alleles is:

TOK, pg 142When Mendel proposed his ideas about “factors” (genes) controlling inherited traits, scientists were not eager to adopt his theories. It was not until many decades later, when a new generation of scientists repeated his experiments that the scientific community started to get excited about genetics. What factors influence scientists in their decision to accept or reject new theories?

Some of Mendel’s results seemed too good to be true. His numbers do not show the expected variations that are typically found when breeding plants. What happened?- Omit unexpected results?- Purposefully change numbers (fudging data)? unethical

How can we be sure that modern scientific studies are free from fudged data?

Gametes have only one allele of each gene.

Punnett grids can be used to show how alleles are split and then recombine.

Purpose is to show all the possible combinations of genetic information for a particular trait.

Steps of the Punnett grid method:1. Choose a letter- generally the first letter of the

dominant trait.2. Identify parents’ genotypes.3. Determine gametes-one set across the top, the

other down the left side.4. Draw the Punnett grid.5. Interpret the grid.

Test CrossesTo determine the genotype of an individual

displaying the dominant phenotype. Cross the dominant individual with one

that is homozygous recessive for the trait.If the offspring are a mix of dominant and

recessive traits heterozygousIf the offspring all show dominant trait

homozygous dominant.

Multiple allelesThree or more alleles for a trait. Ex: blood

type

How would it be possible for a couple to have children with all 4 blood types?

Autosomal genetic diseases in humansCaused by recessive alleles and found on one of

the first 22 pairs of chromosomes.Examples:

Albinism: body is unable to produce/distribute melanin, which is responsible for pigment.

Cystic fibrosis: affects the cells that produce mucus, sweat and digestive juices.

PKU: causes the amino acid phenylalanine to build up in your body.

Sickle cell: A group of disorders that cause red blood cells to become misshapen and break down

Tay Sachs: rare inherited disorder that progressively destroys nerve cells (neurons) in the brain and spinal cord.

Thalassemia: blood disorder in which the body makes an abnormal form of hemoglobin

Sex linkageAny genetic trait whose gene has a locus on the

X or Y chromosome = sex linked.Sex-linked recessive alleles are rare, usually on

X chromosome.Extremely rare to get two X chromosomes with

the recessive allele together, which is why women rarely suffer from these diseases.

Women are the only gender that can be carriers for sex-linked diseases.

Examples:Color blindness (r), hemophilia (r), Duchene

muscular dystrophy (r), white eye color in fruit flies (r), calico fur color in cats.

Mutations and cancerExposure to radiation and/or carcinogens can

modify the genetic code.Base pairs can be knocked out of place, causing

a change in the genetic code. This change is passed on through replication.The mutation may be benign, but it may be

harmful leading to replication of dysfunctional proteins/cells.

VideoAtomic Bomb in Hiroshima: Health Effects