6.1 KEY CONCEPT Gametes have half the number of

chromosomes that body cells have.

You have body cells and gametes.

• Body cells are also called somatic cells.

• Germ cells develop into gametes.– in ovaries and testes– have DNA

body cells sex cells (sperm) sex cells (egg)

• Your body cells have 23 pairs of chromosomes.– Homologous pairs of

chromosomes have the same structure.

– One from each parent• pairs 1-22 are autosomes• Sex chromosomes, X and Y,

determine gender in mammals

Your cells have autosomes and sex chromosomes.

Diploid v. Haploid• Diploid (2n) cells have two copies of every

chromosome.– Body cells are diploid.– Half the chromosomes come from each parent.

• Haploid (n) cells have one copy of every chromosome.

– Gametes are haploid.– Gametes have 22 autosomes and 1 sex chromosome.

• Chromosome number must be maintained in animals.

• Many plants have more than two copies of each chromosome.

• Mitosis and meiosis are types of nuclear division that make different types of cells.

• Mitosis makesmore diploid cells.

• Meiosis makes haploid cells from diploid cells.

– Meiosis occurs in sex cells. – Meiosis produces gametes.

6.2 KEY CONCEPT During meiosis, diploid cells undergo two cell

divisions that result in haploid cells.

Cells go through two rounds of division in meiosis.

• Why is meiosis important?– reduces chromosome number – creates genetic diversity.

• Meiosis I and meiosis II each have four phases, similar to those in mitosis.

homologous chromosomes

sisterchromatids

sisterchromatids

– Homologous chromosomes are similar but not identical.

– Sister chromatids are copies of the same chromosome.

• Meiosis I occurs after DNA has been replicated.• Meiosis I divides homologous chromosomes in

four phases.

Crossing over during meiosis increases genetic diversity.

• Crossing over is the exchange of chromosome segments between homologous chromosomes.

• Meiosis II divides sister chromatids in four phases.

• DNA is not replicated between meiosis I andmeiosis II. Why not?

• Meiosis differs from mitosis in significant ways.

1. Meiosis has two cell divisions while mitosis has one.

2. In mitosis, homologous chromosomes never pair up.

3. Meiosis results in haploid cells; mitosis results in diploid cells.

Haploid cells develop into mature gametes. • Gametogenesis is the

production of gametes.

– Sperm become streamlined and mobile.

– Sperm primarily contribute DNA to an embryo.

– Eggs contribute DNA, cytoplasm, and organelles to an embryo.

– The body gets rid of polar bodies

Summary

• What is the major difference between metaphase I and metaphase II?

• How are homologous chromosomes and sister chromatids different?

• List the key differences between meiosis I and II.

• Why is an egg cell much larger than sperm?

6.3 KEY CONCEPT Mendel’s research showed that

traits are inherited as discrete units.

Mendel laid the groundwork for genetics.

• Traits-distinguishing characteristics that are inherited.

• Genetics is the study of biological inheritance patterns and variation.

• Gregor Mendel showed that traits are inherited as discrete units.

• Many in Mendel’s day thought traits were blended.

Mendel’s data revealed patterns of inheritance.

• Mendel made three key decisions in his experiments.– use of purebred plants– control over breeding– observation of seven

“either-or” traits

• Mendel’s experiment:

• Mendel observed patterns in the first and second generations of his crosses.

• Mendel drew three important conclusions.– Traits are inherited as discrete units.– The last two are the Law of Segregation:

• Organisms inherit two copies of each gene, one from each parent.

• The two copies segregateduring gamete formation.

purple white

6.4 KEY CONCEPT Genes encode proteins that

produce a diverse range of traits.

The same gene can have many versions.

• A gene is a piece of DNA that directs a cell to make a certain protein.

• Each gene has a locus

• An allele is any alternative form of a gene occurring at a specific locus on a chromosome.

– Each parent donates one allele for every gene.

– Homozygous v. Heterozygous

Genes influence the development of traits.

• All of an organism’s genetic material is called the genome.

• A genotype refers to the makeup of a specific set of genes.

• A phenotype is the physical expression of a trait.

• Alleles can be represented using letters.– A dominant allele is

expressed when at least one allele is dominant.

– A recessive allele is expressed when two copies are present.

– Dominant alleles = uppercase letters;

– recessive alleles = lowercase letters.

• Both homozygous dominant and heterozygous genotypes yield a dominant phenotype.

• Most traits occur in a range and do not follow simple dominant-recessive patterns.

6.5 KEY CONCEPT The inheritance of traits follows

the rules of probability.

Punnett squares illustrate genetic crosses. • The Punnett square is a

grid system for predicting all possible genotypes resulting from a cross.– The axes represent

the possible gametesof each parent.

– The boxes show thepossible genotypesof the offspring.

• The Punnett square yields the ratio of possible genotypes and phenotypes.

A monohybrid cross involves one trait. • Mono = One• Monohybrid crosses examine the inheritance of only

one specific trait.– homozygous dominant (two capital letters)-homozygous

recessive (two lower case letters): all heterozygous genotypes

What would all of these plants look like? What would their phenotype be?

Heterozygous-Heterozygous—Genotype = 1:2:1 homozygous dominant: heterozygous: homozygous recessivePhenotype = 3:1 dominant:recessive

• heterozygous-homozygous recessive—– Genotype = 1:1 heterozygous:homozygous recessive; – Phenotype = 1:1 dominant:recessive

The Unknown• What if you have an unknown

genotype?• A testcross is a cross between an

organism with an unknown genotype and an organism with the recessive phenotype.

• You have brown eyes but that means you have two possible genotypes– BB or Bb– If you cross with someone with blue eyes

and have a child with blue eyes, what must your genotype be?

Problem 1

• Use the information from the personal traits lab to set up the following Punnett Square. Predict the phenotypic and genotypic offspring of a cross between a person heterozygous (hybrid) for a widow’s peak and a person homozygous (purebred) for no widow’s peak.

Problem 2

• In pea plants, yellow peas are dominant over green peas. Predict the phenotypic and genotypic outcome of a cross between two plants heterozygous for yellow peas.

Problem 3

• In pea plants, round peas are dominant over wrinkled peas. Use a Punnett square to predict the phenotypic and genotypic outcome of a cross between a plant homozygous for round peas and a plant homozygous for wrinkled peas.

Problem 4

• Write your own problem using one of the traits from your personal traits activity. Tell the genotypes of each parent and then show the Punnett square that would result.

A dihybrid cross involves two traits. • Mendel’s dihybrid crosses

with heterozygous plants yielded a 9:3:3:1 phenotypic ratio.

• Mendel’s dihybrid crosses led to his second law,the law of independent assortment.

• The law of independent assortment states that allele pairs separate independently of each other during meiosis.

Heredity patterns can be calculated with probability.

• Probability is the likelihood that something will happen.• Probability predicts an average number of occurrences,

not an exact number of occurrences.

• Probability =

number of ways a specific event can occurnumber of total possible outcomes

• Probability applies to random events such as meiosis and fertilization.

7.1 KEY CONCEPT The chromosomes on which genes are located

can affect the expression of traits.

Two copies of each autosomal gene affect phenotype.

• Mendel studied autosomal gene traits, like hair texture.

• Mendel’s rules of inheritance apply to autosomal genetic disorders.

– A heterozygote for a recessive disorder is a carrier. – Disorders caused by dominant alleles are uncommon.

(dominant)

Males and females can differ in sex-linked traits.

• Genes on sex chromosomes are called sex-linked genes. – Y chromosome genes in mammals are responsible for male

characteristics.– X chromosome genes in mammals affect many traits.

• Male mammals have an XY genotype.– All of a male’s sex-

linked genes are expressed.

– Males have no second copies of sex-linked genes.

• Female mammals have an XX genotype.– Expression of sex-linked genes is similar to

autosomal genes in females.– X chromosome inactivation randomly “turns

off” one X chromosome.

KEY CONCEPT

Phenotype is affected by many different factors.

• In incomplete dominance, neither allele is completely dominant nor completely recessive.– Heterozygous phenotype is intermediate between the two

homozygous phenotypes

– Homozygous parental phenotypes not seen in F1 offspring

Phenotype can depend on interactions of alleles.

• Codominant alleles will both be completely expressed.– Codominant

alleles are neither dominant nor recessive.

– The ABO blood types result from codominant alleles.

• Many genes have more than two alleles.

Many genes may interact to produce one trait.

• Polygenic traits are produced by two or more genes.

Order of dominance: brown > green > blue.

• An epistatic gene is a single gene that, if present, overrides all other genes– Example: Mouse fur and Albinism

The environment interacts with genotype.

• Phenotype is a combination of genotype and environment.

• The sex of sea turtles depends on both genes and the environment

• Height is an example of a phenotype strongly affected by the environment.