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Biology. Meiosis. How are chromosomes classified?. Autosomes - 1 st 22 pairs of chromosomes Sex chromosomes- last set of chromosomes that determine gender Females only carry X chromosomes in egg Males carry X or Y in sperm If XX- girl If Xy- boy.

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How are chromosomes classified
How are chromosomes classified?

  • Autosomes- 1st 22 pairs of chromosomes

  • Sex chromosomes- last set of chromosomes that determine gender

    • Females only carry X chromosomes in egg

    • Males carry X or Y in sperm

    • If XX- girl

    • If Xy- boy

What are homologous chromosomes
What are homologous chromosomes?

  • Two chromosomes that both carry genes controlling the same inherited trait.

  • One comes from father, one from mother

  • Have same banding pattern

    • Matching bands = matching traits

What is the difference between dominant & recessive alleles?

  • You have 2 alleles for each gene- one from mom & one from dad

  • Dominant allele (trait) =

    capital letter

  • Recessive allele (trait) = lowercase letter

  • EX: Brown eyes (B) dominant over blue (b)

  • Possible gene combinations could be

    • BB = brown eyes (homozygous dominant)

    • Bb = brown eyes (heterozygous)

    • bb = blue eyes (homozygous recessive)

What is the difference between diploid haploid
What is the difference between DIPLOID & HAPLOID?

  • Diploid (2n) = 2 sets of homologous chromosomes.

    • Human somatic cells = 46

  • Haploid (n) = 1 set of homologous chromosomes

    • Human gametes = 23

What is meiosis
What is meiosis?

  • Produces gametes.

  • Reduces the number of chromosomes in the cell so that when gametes combine they have the correct number of chromosomes.

  • 2 major steps:

    • Meiosis I- homologous chromosomes separate

    • Meiosis II- sister chromatids separate (like mitosis)

Meiosis i

  • Interphase- DNA replicates

  • Prophase I-

    • Chromatin coils up

    • Homologous chromosomes pair up and form a tetrad (4 sister chromatids)

    • Nucleus and nucleolus disappear

    • Spindle forms and captures tetrad of chromosomes

  • Metaphase I-

    • Tetrad lines up at middle of cell

  • Anaphase I-

    • Tetrad splits. One chromosome goes to one pole, other goes to other pole

  • Telophase I-

    • Cell splits, nucleus and other parts reform.

Meiosis ii

  • This is essentially the same as mitosis

  • The two cells from meiosis I go through these steps at the same time.

  • Prophase II

    • Spindle forms, chromosomes attach

  • Metaphase II

    • Chromosomes line up at the middle of cell

  • Anaphase II

    • Sister chromatids are pulled apart

  • Telophase II

    • Each cell splits into 2 more cells creating a total of 4 cells.

How is meiosis different in males females

In females:

In ovaries


1 mature egg,

3 polar bodies which break down

Women born with all eggs they will have.

Egg is much larger

Have all X chromosomes

Has no method of movement

In males:

In testes

Produces:4 mature sperm

Males begin to produce sperm after puberty, produced constantly until death

Much smaller than egg

May have X or Y chromosomes

Have flagella to move

How is meiosis different in males & females?

What is independent assortment

  • Chromosomes can line up in any order before separating during metaphase I.

  • This leads to some gametes getting only mother genes and some only father genes.

  • This can also lead to some gametes getting mixture of mother and father’s genes.

  • See figure 8.16 p. 143

What is crossing over

  • Sometimes sister chromatids can exchange pieces of corresponding chromosome creating a new combination of traits.

  • Figure 8.18B p. 145

Describe karyotyping
Describe karyotyping

  • Picture of chromosomes are taken from a cell sample, cut out and matched up in pairs

  • Humans have 23 pairs of chromosomes

  • Karyotypes can be used to determine if genetic disorder is present

  • If too many are present can indicate Down’s syndrome

  • If some are missing can indicate Turner’s syndrome

Errors in meiosis

  • Nondisjunction:

    • Members of a chromosome pair fail to separate. Resulting daughter cells have too many chromosomes.

    • Examples:

    • Trisomy 21- extra #21: down’s syndrome

    • Klinefelter’s syndrome- XXY

    • XYY male- taller than average, increased testosterone levels

    • Turner’s syndrome- XO, females with only one sex chromosomes.

    • See figure 8.21 A, B, C p. 148