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Summary Silverthorn Chapter 26 - Reproduction and development
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Summary Silverthorn Chapter 26
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SILVERTHORNCHAPTER 26 – REPRODUCTION AND
DEVELOPMENT
Puberty is the period when a person makes the transition from being non-reproductive to
reproductive. Pseudo hermaphrodites are men who have internal sex organs of a male, but inherit
a gene that causes a deficiency in one of the male hormones. Therefore, they are born with external
genitalia that appear feminine, and they are raised as girls. At puberty, they begin to secrete more
male hormones. As a result, they develop some, but not all, of the characteristics of men. Humans
are sexually dimorphic, meaning that males and females are physically distinct.
SEX DETERMINATION
The male and female sex organs consists of three sets of structures:
1. Gonads – the organs that produce gametes, the eggs and sperm that unite to form a new
individual. The male gonads are the testes, which produce sperm (spermatozoa). The
female gonads are the ovaries, which produce eggs or ova. The undifferentiated gonadal
cells destined to produce eggs and sperm are called germ cells.
2. Internal genitalia – consist of accessory glands and ducts that connect the gonads with
the outside environment
3. External genitalia – include all external reproductive structures
Each nucleated cell of the body except eggs and sperm contain 46 chromosomes. This set is called
diploid number, because the chromosomes occur in pairs: 22 matched (homologous) pairs of
autosomes plus one pair of sex chromosomes.
Autosomal chromosomes direct development of the human body form and of variable
characteristics such as hair colour and blood type. Sex
chromosomes determine the development of in- and external
sex organs. The X chromosome is larger than the Y and
includes many genes that are missing from Y.
Eggs and sperm are haploid cells with 23 chromosomes, one
from each matched pair and one sex chromosome. When eggs
and sperm unite, the resulting zygote then contains a unique
set of 46 chromosomes, with one chromosome of each
matched pair coming from the mother and the other from the
father.
SEX CHROMOSOMES DETERMINE GENETIC SEX
Genetic females are XX and genetic males XY. If sex chromosomes are abnormally distributed at
fertilization, the presence or absence of a Y chromosome determines whether development
proceeds along male or female lines. A XXY zygote will become male. A zygote that inherits only
a Y chromosome (YO) will die, because the larger X chromosome contains essential genes that
are missing from the Y chromosome. In contrast, a zygote that only gets one X chromosome (XO;
Turner’s syndrome) will develop into a female. However, two X chromosomes are needed for
normal female reproductive function.
Silverthorn – chapter 26: Reproduction and development Page 1 of 21
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