HEREDITY UNIT NOTES

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HEREDITY UNIT NOTES

Targets 11, 12, 13

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Heredity Unit Objectives 11-13

H.11 I can explain the main events in each of the eight stages of meiosis.

H.12 I can describe the factors that result in genetic variation, including mutations, crossing-over,

independent assortment, and sexual reproduction.

H.13 I can describe the history of our understanding of genetics, including the significance of the

works of Mendel, Griffith, Franklin, and Watson and Crick.

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TARGET H.11

I can explain the main events in each of the eight stages of

meiosis.

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A Little Review…

 Humans have 23 pairs of chromosomes, for a total of 46.

 In each pair, one chromosome comes from mom and one comes from dad.

 Both members of the chromosome pair are the same size, shape, and carry the same genetic information (have the

same genes on them).

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Sets of Chromosomes

 The 46 chromosomes in humans are 2 sets, or 2 pairs, of 23.

 The 23 pairs of chromosomes (one from mom, one from dad) are known as

homologous chromosomes.

 Cells that are not reproductive (any cell except egg or sperm) are called

somatic cells and have all 46 chromosomes.

 Having both sets of 23 chromosomes is called diploid (2n).

 Therefore, all somatic cells are diploid.

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Gametes

 Gametes are reproductive cells, or egg and sperm.

 Gametes have only one set of 23 chromosomes.

 Having only one set of 23 chromosomes is known as haploid (n).

 Why do they only have 23 chromosomes, not 46?

 That way, when the egg and sperm fuse, you get the normal amount of 46.

Otherwise, each generation would double the chromosomes.

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Sex Chromosomes

 Of the 23 pairs of chromosomes, 22 pairs are called autosomes, and one pair

determines the sex, or gender, of the individual.

 Autosomes are chromosomes not directly involved in determining the gender of an individual.

 The sex chromosomes in humans are known as the X and Y chromosomes.

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Determining Gender

 Females = XXMales = XY

 The sperm determines the sex of the offspring.

 The egg can only give an X chromosome, but the sperm can give an X or a Y

chromosome.

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Meiosis

 Multicellular organisms produce

gametes through the process of meiosis.

 In humans and many other organisms, these cells are called ova (eggs) and sperm.

 When an ovum and a sperm combine, a new organism is formed with a full set of chromosomes, half of which came from the mother/half of which came from the father.

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It’s all about Half

 Meiosis is a type of cell division where one cell divides to form new cells, each of which have half the number of

chromosomes as the original (the new cells are haploid).

 So the parent cell is diploid, but the daughter cells are haploid.

 There are two main stages of meiosis:

Meiosis I and Meiosis II.

 Each half of meiosis has four stages:

prophase, metaphase, anaphase, and telophase.

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Prophase I

 Homologous pairs will temporarily attach together to form a structure called a tetrad.

 Otherwise, this phase is similar to prophase of mitosis.

 Chromatin

coils/condenses to form chromosomes.

 Nuclear membrane disappears.

 Centrioles start to make spindle fibers.

Tetrads

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Crossing Over and Recombination

 Sometimes, the tetrads actually

exchange some of their DNA in a

process called crossing over.

 If this happens, it will happen during Prophase I, while tetrads are

connected.

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Homologous chromosomes

 Alleles reside at the same location or locus on homologous chromosomes.

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Crossing Over and Recombination

 Chromatids exchange homologous sections carrying alleles, producing recombinant

daughter chromosomes with a different combination of alleles.

 Crossing over is a source of genetic variation – a key factor in evolution!

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Metaphase I

 Tetrads, not individual

chromosomes, line up on the equator of the cell.

 Spindle fibers attach to the

centromeres of the chromosomes.

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Anaphase I

 Spindle fibers pull tetrads apart.

 Each entire

chromosome is pulled to opposite ends of the cell.

 At the end of this phase,

chromosome

number is now cut in half!

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Telophase I

 Cell begins to pinch.

 Cell usually goes directly into

Prophase II, but if

not, chromosomes will uncoil, nuclei will

reform, and spindle fibers will break

down.

 Each daughter cell has half of the number of chromosomes as the original.

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Summary of Meiosis II

 The mechanics of meiosis II are identical to mitosis.

 Each haploid daughter cell produced by meiosis I divides again, so that four

haploid cells are produced altogether.

 In spermatogenesis, all four cells survive to produce viable sperm.

 In oogenesis, only one cell survives to produce a viable ovum.

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Overview of Meiosis

 Tetrads form (Pro I)

 Crossing over can occur (Pro I)

 Tetrads line up (Meta I) and are separated (Ana I)

 Daughter cells are haploid.

 Produces gametes (egg and sperm)

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Differences Between Meiosis and Mitosis

 Mitosis doesn’t have tetrads.

 Mitosis doesn’t have crossing over.

 Chromosomes are separated in mitosis, while tetrads are separated in meiosis I

 Daughter cells are diploid in mitosis, haploid in meiosis.

 Mitosis produces more somatic cells, meiosis produces gametes.

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Why?

MITOSIS MEIOSIS

 Mitosis occurs for growth

 Mitosis occurs for repair

 Meiosis

occurs for

reproductio n.

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IDENTIFYING PHASES OF MITOSIS OR

MEIOSIS

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Step 1 – Which Phase?

 Although there are differences between each, learn and understand the MAIN event of each phase (PMAT) because these are the same no matter what.

 Prophase – nucleus will be disappearing and chromosomes will be visible.

 Metaphase – chromosomes will be lined up

 Anaphase – something will be pulled apart

 Telophase – cell will be pinching

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Step 2 – Mitosis, Meiosis I, or Meiosis II?

 Now that you know whether it’s P, M, A, or T, you need to decide if that’s from mitosis, meiosis I, or meiosis II.

 Learn the key differences between each.

(Look back at the comparison picture).

 Now label your picture!

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Step 3 – Look for Dead Giveaways

 There are some dead giveaways.

 If there are two cells

 it must be Meiosis II.

 If there’s one cell but with pairs

 it must be Meiosis I.

 If there’s one cell and it has separate chromosomes, no pairs

 it’s going to be mitosis.

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Try These!

1.

2. 3.

4. 5.

Metaphase II

Anaphase Prophase

Anaphase I Telophase I

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TARGET H.12

I can describe the factors that result in genetic variation, including mutations,

crossing-over, independent

assortment, and sexual reproduction.

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Why is Genetic Variation So Important?

Differences in DNA and genes (genetic variation) is a key factor in

evolution (changes within populations of living organisms).

Without changes,

especially in response to a change in the

environment, species may go extinct!

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Copyright Pearson Prentice Hall

Genetic Variation

 Many genes have at least two forms, or alleles.

 All organisms have genetic variation that is

“invisible” because it involves small differences in biochemical processes.

 An individual organism is heterozygous for many genes.

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Variation and Gene Pools

Variation and Gene Pools

A population is a group of individuals of the same species that interbreed.

A gene pool consists of all genes, including all the different alleles, that are present in a population.

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The relative frequency of an allele is the number of times the allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur.

Relative frequency is often expressed as a percentage, and it is not related to whether an allele is dominant or recessive.

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Gene Pool for Fur Color in MiceSample Population Frequency of Alleles

allele for brown fur

allele for black fur

When scientists determine whether a population is evolving, they may look at

the sum of the population’s alleles, or its gene pool. This diagram shows the gene pool for fur color in a population of mice.

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In genetic terms, evolution is any

change in the relative frequency of alleles in a population.

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Sources of Genetic Variation

 The main sources of genetic variation are mutations and the genetic

shuffling that results from sexual reproduction.

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Mutations

 A mutation is any change in a sequence of DNA.

 Mutations occur because of mistakes in DNA replication or as a result of mutagens

(radiation or chemicals) in the environment, for example.

 Mutations do not always affect an organism’s phenotype.

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Gene Shuffling

Most heritable

differences are due to gene

shuffling.

So how do genes

get “shuffled”?

.

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Sexual reproduction results in gene shuffling, due to:

• random fertilization of gametes

(which sperm fertilizes the egg is a random event)

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• The independent assortment of homologous chromosomes that occurs in meiosis

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Law of Independent Assortment: For Homologous Chromosomes

 Each pair of chromosomes segregates independently of other pairs of

chromosomes during gamete formation.

 Which means you could get your mom’s hair color and not get her eye color (or vice

versa). Traits on different pairs are inherited INDEPENDENTLY of one another.

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Law of Independent Assortment

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• Crossing over – the exchange of chromosome segments between homologous chromosomes during meiosis

• Results in new combinations of genes

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Any Questions?