Name_____________________________________
Unit 2 Lesson 2
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Before sexual reproduction can take place, each parent produces ________________________.
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Sex cells have ______________ of the genetic information that body cells have.
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When genetic information from two parents combines, the offspring will have one full set of
genetic information.
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In body cells, chromosomes are found in pairs of _________________________ chromosomes,
which have the same structure and size.
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____________________________ chromosomes carry the same genes. However, some may be
different versions of the genes.
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One chromosome pair is made up of sex chromosomes, which control the development of
sexual characteristics.
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In humans, the sex chromosomes are called ___ and ___ chromosomes.
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Cells with a pair of every chromosome are called ________________________.
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Most human body cells contain ________ chromosomes.
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Sex cells are also known as ____________________. __________________ have only half the
usual number of chromosomes.
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Gametes are _____________________, meaning they have one chromosome from each
homologous pair. Gametes are made in the reproductive organs.
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When sex cells combine during sexual reproduction, the resulting cell has 46 chromosomes: half
from the _____________ and half from the _______________.
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____________________ produces two new cells that contain exact copies of the chromosomes
in the parent cell.
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A different kind of cell division is needed to produce sex cells.
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____________________ is the type of cell division that produces haploid sex cells such as sperm
or egg cells.
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When an egg is _______________________ by a sperm cell, a new diploid cell forms.
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Meiosis has two parts: meiosis I and meiosis II.
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Recall that homologous chromosomes have the same genes but are not exact copies of each
other.
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Before meiosis I begins, each chromosome is ______________________. Each half is called a
chromatid.
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Chromatids are connected by _______________________________.
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During meiosis I, pairs of homologous chromosomes and sex chromosomes split apart into two
new cells.
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In _________________________, duplicated homologous chromosomes pair up.
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In _________________________, the homologous chromosome pairs line up in the middle of
the cell.
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In _________________________, the homologous chromosomes separate from their partners
and move to opposite ends.
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In _________________________ and cytokinesis, the nuclear membranes re-form and the cell
divides into two cells. The chromatids are still joined.
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Meiosis II involves both of the new cells that formed during meiosis I.
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The new cells divide during meiosis II.
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Meiosis II results in _____________ haploid sex cells.
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In males, the four sex cells develop into ________________cells.
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In females, the sex cells become _______________ cells.
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In females of some species, three cells are broken down and only one haploid cell becomes an
egg.
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In ________________________, the nuclear membrane breaks apart.
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In ________________________, the chromosomes line up in the middle of each cell.
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In ________________________, the chromatids are pulled apart and move to opposite sides.
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In ________________________ and cytokinesis, the nuclear membranes re-form and the cells
divide. Each cell is haploid.
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Only cells that will become sex cells undergo meiosis. All other cells divide by mitosis.
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In meiosis, chromosomes are copied once, and the nucleus divides _____________________.
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In mitosis, the chromosomes are copied once, and the nucleus divides once.
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Meiosis produces haploid cells. Mitosis produces diploid cells.
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Down syndrome is a genetic disease caused by an error during meiosis.
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The chromatids in chromosome 21 do not separate, so one of the sex cells gets an extra copy of
chromosome 21.
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Down syndrome causes a number of health problems and learning difficulties, but many people
with Down syndrome have fulfilling lives.