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Heredity  the transmission of traits from one generation to the next
Variation  when offspring differ somewhat from their parents and siblings
Genetics  the scientific study of heredity and hereditary variation
Genes  segments of DNA that code for individual traits/proteins; parents endow their offspring with this coded information (genes); genetic information is transmitted as specific sequences of the four nucleotides in
DNA.
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Most genes program cells to synthesize specific enzymes and other proteins whose cumulative action produces an organism’s inherited traits.
Gametes  reproductive cells; haploid (one copy of each chromosome); egg and sperm; made during meiosis in the gonads (ovary/testes)
Fertilization  fusion of egg and sperm
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After fertilization genes from both parents are present in the nucleus of the fertilized egg, or zygote .
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ASEXUAL (MITOSIS) →
- Makes SOMATIC cells
- one diploid to two diploid cells
- identical to parent
- ONE parent donates genetic information
SEXUAL (MEIOSIS) →
- makes GAMETES
- One diploid to four haploid cells
- All new cells are
GENETICALLY
DIFFERENT from each parent and each other
- TWO parents contribute genetic information
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Diploid  TWO sets of each chromosome pair
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Humans have a diploid number of 46; therefore each somatic cell
(body cell) has 46 chromosomes
This is represented by 2n
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These chromosome pairs are called homologous chromosomes (see next slide)
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Cells made by MITOSIS are diploid
Haploid  ONE set of each chromosome pair
Humans have a haploid number of 23; therefore each gamete
(sperm/egg) have 23 chromosomes
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This is represented by n
Cells made by MEIOSIS are haploid
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Diploid organisms have homologous chromosomes. These chromosomes are the same size, shape, carry genes for the same traits, and have the centromere in the same spot. However, the genes can be different!
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For example, the maternal chromosome (the one you got from mom) can have a brown eye gene and the paternal chromosome can have a blue eye gene
The EXCEPTION to the homologous chromosome pattern are the sex chromosomes , X and Y
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Females = XX Males = XY
The other 22 pairs of chromosomes are called autosomes .
It is crucial to understand the differences among homologous chromosomes, sister chromatids, nonsister chromatids, and chromosome sets.
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Down Syndrome Karyotype
A karyotype is a picture of all the chromosomes of an organism. The chromosomes are usually ordered by size and can give clues about genetic disorders. They can be done on babies before they are born by amniocentesis or CVS – chorionic villi sampling.
This can tell doctors if the person has an extra chromosome or is missing one, or if they have some other type of chromosomal disorder (deletion, duplication, etc).
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The human life cycle begins when a haploid sperm cell fuses with a haploid ovum (egg), this joining of cells is called syngamy .
The union of these gametes, culminating in the fusion of their nuclei, is fertilization. (in other words, syngamy precedes fertilization)
The fertilized egg ( zygote ) is diploid because it contains two haploid sets of chromosomes bearing genes from the maternal and paternal family lines.
As a person develops from a zygote to a sexually mature adult, mitosis generates all the somatic cells of the body.
Gametes develop from specialized cells called germ cells in the gonads .
Gametes undergo the process of meiosis , in which the chromosome
number is halved.
Fertilization restores the diploid condition by combining two haploid sets of chromosomes.
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Life Cycle  generation to generation sequence of stages in the reproductive history of an organism
Fertilization and meiosis alternate in all sexual life cycles, in plants, fungi, protists, and animals.
Gametes (HAP) → Fertilization
→ Zygote (DIP) → Mitosis and
Development → Meiosis in
Gonads → Gametes (HAP)
Adult form is Diploid…meiosis is done only to create haploid gametes
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- This life cycle includes two multicellular stages , one haploid and one diploid .
- The multicellular diploid stage is called the sporophyte .
Meiosis in the sporophyte produces haploid spores .
- Unlike a gamete, a haploid spore doesn’t fuse with another cell but rather divides by mitosis to form a multicellular haploid gametophyte stage.
Gametes produced via mitosis by the gametophyte fuse to form the zygote , which grows into the sporophyte by mitosis.
- In this type of life cycle, the sporophyte generation produces a gametophyte as its offspring, and the gametophyte generation produces the next sporophyte generation .
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“Alternation of
Generations”
Although the three types of sexual life cycles differ in the timing of meiosis and fertilization, they share a fundamental result: genetic variation among offspring
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Many steps of meiosis resemble steps in mitosis.
Both meiosis and mitosis are preceded by the duplication of chromosomes . (during interphase)
In meiosis, there are two consecutive cell divisions , meiosis I and meiosis II , resulting in four daughter cells.
The first division, meiosis I, separates homologous chromosomes.
The second division, meiosis II, separates sister chromatids.
The four daughter cells at the end of meiosis have only HALF as many chromosomes as the original parent cell.
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Meiosis I is preceded by interphase , in which the chromosomes are duplicated to form sister chromatids.
Two genetically identical sister chromatids make up one duplicated chromosome .
The sister chromatids are closely associated all along their length.
This association is called sister chromatid cohesion . (Recall this from Chapter 12!)
In contrast, the two chromosomes of a homologous pair are individual chromosomes that were inherited from different parents .
Homologous chromosomes appear to be alike, but they may have different versions of genes , each called an allele , at corresponding loci.
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Synapsis = Formation of Tetrads
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When the homologous chromosomes come together they are held together by the synaptonemal complex ; this process is called synapsis
Crossing over also happens at this stage
Occurs during Prophase I
 Ignore the poor English…
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Crossing over occurs during Prophase I (only
Meiosis I – not meiosis II because tetrads are not present in Meiosis II) of meiosis. The homologous chromosomes of the tetrads can switch positions and the genes can be exchanged. This is one of the processes that leads to genetic variation .
Crossing over occurs between NON-
SISTER CHROMATIDS . The location where crossing over happens is called the chiasmata . Crossing over can happen 2-3 times per chromosome.
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Meiosis I → homologous chromosomes are separated
Meiosis II → sister chromatids are separated
Meiosis = Two divisions; creates 4 new haploid cells that are genetically different from the parent cell.
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Making the tetrads (synapsis) and crossing over between non-sister chromatids; occurs during
Prophase I
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In mitosis this step is just the individual chromosomes on the metaphase plate, not the tetrads
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In Anaphase I, the tetrads separate; in mitosis the sister chromatids separate during anaphase
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Although MUTATIONS are the
ORIGINAL SOURCE of genetic diversity , there are three factors that contribute to increasing genetic variation during meiosis:
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- Independent Assortment
- Crossing Over
- Random Fertilization
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If n = 3, there are 2 3 = 8 possible combinations.
For humans with n = 23, there are 2 23 , or about 8.4 million possible combinations of chromosomes.
Independent assortment means that the maternal chromosomes and the paternal chromosomes can line up any way possible. This contributes to genetic variation.
To determine how many possibilities there are, use the following equation:
N = haploid number
2 N = number of combinations
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Produces RECOMBINANT
CHROMOSOMES . This process combines genes inherited from each parent. Recall: the location of crossing over is called chiasmata
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Charles Darwin recognized the importance of genetic variation in evolution.
As the environment changes, the population may survive if some members can cope effectively with the new conditions.
Mutations are the original source of different alleles, which are then mixed and matched during meiosis.
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