Introduction to Genetics

Chapter 11

The Work of Gregor Mendel

Genetics• The scientific study of heredity.

Vocabulary to Know• Trait: a specific characteristic varying

among individualsex: eye color

• Gene: a portion of DNA determining a trait; found on the chromosomes

ex: the gene for eye color

Vocabulary (continued)• Hybrid: the offspring of

two parents with different traits

• Gametes: reproductive cells (aka: sex cells; sperm & egg)

Allele: the form of a gene• Allele: the different forms of a

gene* Represented by a letter.ex: Widow’s Peak = W* Organisms have two alleles for each trait, one inherited from the mother and one from the father.ex: WW

Forms of AllelesDominant Allele: trait will be expressed with

only one copy present* Represented with capital letters.ex: W (Widow’s peak)

Recessive Allele: trait will only be expressed when no dominant alleles are present* Represented with lower case letters.ex: w (No widow’s peak)

Gregor Mendel• Considered the father of genetics.

• Mid-1800s

• Monk who experimented with pea plants in the monastery garden.

Mendel’s Conclusions1. Inheritance is determined by

factors passed from one generation to the next.

2. Principle of dominance: Some alleles are dominant and others are recessive.

Mendel’s Conclusions3. Segregation: A gamete carries

only one copy of each gene.

4. Principle of independent assortment: Genes for different traits segregate independently from one another.

Applying Mendel’s Principles

Allele Combinations• Homozygous: both alleles are the

sameex: WW or ww

• Heterozygous: alleles are differentex: Ww (capital letter is

always 1st!)

• Genotype: the actual allele combination; what the genes say

ex: WW, Ww, ww

• Phenotype: the trait observedex: Widow’s Peak

Punnett Squares• Used to predict the genotypes of

offspring when the genotypes of both parents is known.

Other Patterns of Inheritance

Incomplete Dominance The heterozygous phenotype (Ww) is

somewhere between both homozygous phenotypes (WW & ww).

Ex: pink flowers

Codominance• Both alleles contribute to the phenotype; neither allele is dominant .

•Ex: roan cattle

Multiple Alleles More than two allele possibilities.

Ex: blood type

Polygenic Traits Traits controlled by more than one

gene.

Ex: skin color

Meiosis

Words to Know Haploid: “one set”; cells that contain one set

of each chromosome (gametes); 1n Example: in humans n=23, so human

haploid cells (gametes) have 1(23) or 23 total chromosomes

Diploid: “two sets”; cells that contain two sets of each chromosome ; 2n Example: in humans n=23, so human

diploid cells have 2(23) or 46 total chromosomes

Homologous Chromosomes

The pair of chromosomes that have the genes for the same traits. A copy is inherited from each

parent.

Meiosis A type of cell division that produces

gametes containing half the number of chromosomes as a body cell. Contains two separate rounds of

division, called Meiosis I & Meiosis II. For example, during meiosis in humans,

diploid cells with 46 chromosomes are divided into 4 haploid sex cells (gametes) with 23 chromosomes each.

Animation/

Phases of

Meiosis

Interphase: cell grows, replicates its chromosomes and prepares to divide.

Prophase I: chromosomes take shape; homologous chromosomes pair up, forming a tetrad Crossing over can occur!

Chromosomes touch & exchange genes. This can mix up linked genes. (ones found on the same chromosome that occur together).

Metaphase I, Anaphase I, Telophase I

•Chromosomes line up across

the cell’s center.

•Homologous chromosome

pairs separate.

•Nuclear membrane reforms &

cytokinesis occurs.

•Similar to Mitosis, except 4 haploid daughter cells are created.

Meiosis II

Metaphase II: • Chromosomes line up across the cell’s center.

Prophase II:• No Replication occurs!

Meiosis II (continued)

Anaphase II: •Chromosomes separate.

Telophase II: • Nuclear membrane reforms & cytokinesis occurs.

The end result: In males, all 4 of

the daughter cells become sperm.

In females, the division of the cytoplasm is uneven, so only 1 egg is produced. The other three cells, called polar bodies are not used for reproduction.

Genetic Recombination The reassortment of chromosomes and the

genetic information that they carry by segregation and crossing over.

For example: In humans, n = 23, so the number of different

gametes that can be produced is 223 (more than 8 million).

When fertilization occurs, 223 x 223, or 70 trillion different zygotes are possible and this is without accounting for crossing over! Zygote: the fertilized egg

Mitosis vs. MeiosisMitosis Meiosis• Two identical daughter cells are formed.

• Each daughter cell has one complete set of chromosomes. (diploid)

• Four different gamete cells are created (although in females, only 1 is viable).

• Each gamete cell has half of the total number of chromosomes. (haploid)

http://www.pbs.org/wgbh/nova/miracle/divi_flash.html

Linked genes Genes that are located on the

same chromosome & thus, tend to be inherited together.

Human Heredity

Genome: the full set of genetic information carried in an organism’s DNA Humans have 46 chromosomes in

their genome.

Karyotype: a picture of all diploid pairs of chromosomes, arranged in order of decreasing size

Chromosome TypesAutosomes: the remaining 44 human

chromosomes other than the sex chromosomes

Sex chromosomes: the pair of chromosomes determining gender Males have XY. Females have XX.

In female cells, most of the genes in 1 X are randomly switched off (called Barr bodies).

Ex: female calico cats

Sex-linked genes A gene located on a sex

chromosome. Genes on the Y chromosome

are only found in males. Genes on the X chromosome

are found in both genders. For recessive traits on the X,

they are more common in males than females.

Ex: color-blindedness, hemophilia

Pedigree A chart used to show the presence

or absence of a trait within a family.

Human Genetic Disorders

Nondisjunction When homologous chromosomes

fail to separate during meiosis. Creates monosomy and trisomy.

http://learn.genetics.utah.edu/content/chromosomes/diagnose/WHEN A MISTAKE IS

MADE…