Mrs. Degl

Modern Genetics

Gene Linkage: If the genes of two different traits (non-allelic) are located on the same chromosome pair (homologous chromosomes) they are said to be linked, and are therefore inherited together.

Crossing Over: In Meiosis 1(in Prophase) the chromatids of a pair of homologous chromosomes often twist around each other. They can then break, exchange segments, and rejoin. This is called Crossing Over and it produces important variation in offspring, during sexual reproduction.

Mrs. Degl

Mrs. Degl

Sex Determination•In human body cells there are 22 pairs of autosomes (a chromosome that is not a sex chromosome) and 1 pair of sex chromosomes •The sex chromosomes are the X and Y chromosomes•Females have 2 X chromosomes (XX)•Males have 1 X and 1 Y chromosome (XY)•During Meiosis the sex chromosomes (like all of the rest of the chromosomes) separate so each gamete is either X or Y•All females gametes (eggs) will be X•Male gametes (sperm) can be either X or Y•At fertilization the sex of the baby is determined when either the X or Y sperm enters the egg (which is X), resulting in either a girl (XX) or boy (XY)

Mrs. Degl

Sex-linked traits•T.H. Morgan’s work with Drosophila (fruit flies) found that some rare recessive traits appear more often in males than in females.

•He concluded that these traits must be on the X chromosome. These are called sex-linked genes.

•Recessive traits that are sex-linked occur more frequently in males because in females there is usually a normal dominant allele on the other X chromosome to overpower the recessive X chromosome, labeled (X’).

•In males there is not another X chromosome to overpower the recessive one (males are XY), so the recessive phenotype takes over, producing X’Y.

Mrs. Degl

Human Sex-linked Disorders

Hemophilia- disease in which the blood does not clot properly

Colorblindness- inability to see certain colors, most commonly red and green

• Both of these disorders are more common in males than in females because a female will not show the disorder as long as she has one normal gene. Females who are heterozygous for a sex-linked trait are said to be carriers for that trait.

• Females with one normal dominant and one recessive gene are called carriers and have no symptoms, but can pass it along to offspring.

Mrs. Degl

A punnett square for the X-linked characteristic color blindness in humans.

Female carrier XCXc crossed with normal male XCY:

Mrs. Degl

• Changes in genetic material are called mutations• If a mutation occurs in the sex cell, it may be

transmitted to the offspring• Mutations occurring in body cells may be passed

on to new cells of the individual due to mitosis, but will not be transmitted to the offspring by sexual reproduction

Genetic Mutations

There are two groups of mutations:1.Chromosomal Alterations

2.Gene Mutations

Mrs. Degl

1. Chromosomal AlterationsThese involve changes in the structure or number of chromosomes.

One example is Nondisjunction

• One or more pairs of homologous chromosomes fail to separate.

• This results in gametes with more (or less) than their normal haploid chromosome number.

Mrs. Degl

Down Syndrome is caused by the presence of an extra chromosome (# 21).

•It is characterized by mental retardation, an abnormal pattern of palm creases, a flat face, sparse, straight hair, and short stature. •People with Down syndrome have a high risk of having cardiac anomalies, leukemia, cataracts, and digestive blockages.•Life expectancy of Down syndrome individuals is in the middle teens but some live much longer.•The gene responsible for Alzheimer’s is on chromosome 21. Down’s are at increased risk for developing Alzheimer’s.•Down Syndrome is associated with maternal age.

Mrs. Degl

Polyploidy- sometimes all 22 chromosomal pairs fail to separate. The resulting 2n gamete fuses with the normal n gamete, producing a 3n zygote. This is common in plants but rare in humans

Polyploidy in PlantsPolyploidy is a major evolutionary mechanism in plants. Approximately 47% of all flowering plants are polyploid.

Some examples of polyploid plant species are corn, wheat, cotton, sugarcane, apples, bananas, watermelons, and

many flowers

Polyploidy in HumansPolyploids have defects in nearly all organs.

Most die as embryos or fetuses. Occasionally an infant survives for a few days.

Mrs. Degl

Changes in Chromosome Structure

Translocation: when a segment of one chromosome breaks off and reattaches to a non-homologous chromosome.

Addition: when a segment breaks off one chromosome and reattaches to the homologous chromosome.

Inversion: when a segment breaks off and reattaches in reverse on the same chromosome (it is upside-down).

Deletion: when a segment breaks off and does not reattach to any other chromosome.

Mrs. Degl

2. Gene Mutations•A random change in the chemical makeup of the DNA is a gene mutation. The effects of some mutations, such as albinism, are noticeable, but others are not.

•Inheritable mutations are usually harmful.

•Fortunately, most mutations are recessive, and therefore the dominant allele prevails.

•If both parents carry the recessive mutation, than the child will possess the mutation (Sickle Cell Anemia, Tay-Sachs).

•Exposure to chemicals and radiation can increase the rate of mutations. X-rays, UV Rays, Cosmic Rays, formaldehyde, and asbestos are examples of Mutagenic Agents.

Mrs. Degl

Heredity and the Environment

The development of inherited traits can be influenced by environmental factors such as temperature, sunlight, and nutrients.

•Normally, Himalayan Rabbits are white with black ears, nose, tail and feet. The black hair helps the colder areas absorb heat. •When some fur is shaved off the rabbit and that area is kept covered with ice, the new hair grows in black. The artificial temperature change causes the hair to grow in black, so that it can absorb heat to help the rabbit.

Mrs. Degl

Breeders of animals and plants in today's world are looking to produce organisms that will possess desirable characteristics, such as high crop yields, resistance to disease, high growth rate and many other phenotypical characteristics that will benefit the organism and species in the long term.

Selective Breeding

There are 3 methods of selective breeding:

•Artificial Selection•Inbreeding•Hybridization

Mrs. Degl

Artificial Selection: Individuals with the most desirable traits are crossed or allowed to mate in the hopes that their offspring will show the desired traits. Examples are crossing two tall plants or having two sheep with thick wool mate to get tall plants or sheep with very thick wool.

Inbreeding: Two offspring from the same parents can be crossed or allowed to mate to produce desired characteristics. However, the danger of inbreeding is that it is more likely that two recessive traits can be inherited.

Hybridization: Crossing two different species to get desired traits. This is more common in plants but can be done with animals as well. Can these species reproduce?

Mrs. Degl

ZorseLigerHybridization Breeding