Chapter 3

Chapter 3 - Chapter 3: Hereditary Influences on Development...

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Chapter 3: Hereditary Influences on Development General principles: - Genotype: the genes an individual inherits. - Phenotype: ways in which an individual’s genotype is expressed in observable/measurable characteristics. - Conception: moment of fertilization, when a sperm penetrates an ovum, forming a zygote. Zygote, etc.: - One cell; union of sperm and ovum; 1/20 th size of head of a pin. - Zygotic nucleus contains 46 chromosomes (*every body cell nucleus contains this number of chromosomes). - Each chromosome contains thousands of genes (DNA stretches). - With one exception, chromosomes come in matching pairs. - One member of each pair comes from ovum and one from sperm. - Thus, each parent contributes 23 chromosomes to a child. - The ability of DNA to duplicate itself makes it possible for a one- celled zygote to develop into a complex living organism. Growth of zygote + production of body cells: - As zygote moves up fallopian tube toward the uterus, it begins to duplicate through the process of mitosis . - Before each division, the cell duplicates its 46 chromosomes- each set of 23 identical pairs forms a new cell; the process continues. Germ (or sex) cells: - Humans have germ cells whose function is to produce gametes : sperm in males and ova in females. - Germ cell reproduction differs from mitosis in ways that make the resulting gamete able to join with another gamete. - Gametes are produced through the special process of meiosis.
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- Germ cell (46 chromosomes) splits, producing gametes that each contains half the parent cell’s original chromosomes (23) - The duplication of the 46 chromosomes is followed by the process of crossing-over: adjacent duplicated chromosomes cross + break at one or more points along their length, exchanging segments of genetic material– creates new and unique hereditary combinations. - Then the pairs segregate into 2 new cells, each w/46 chromosomes. - Finally, these 2 cells divide so that each gamete contains 23 single (unpaired) chromosomes. More meiosis facts: - During duplication, the original strand and its duplicate are held together by a structure called the centromere. - Each chromosome has both a long arm and a short arm extending from the centromere. - Cross-over sites are called chiasmata. - Average # of cross-over events per meiosis is 42-females, 27-males. - There are specific, nonrandom locations along the chromosome where crossing-over is more likely to occur (“hotspots”). - Crossing-over serves 2 important functions: 1- increase of genetic variability in the population; 2- chiasmata formed during cross- overs tether homologous chromosomes together, which helps prevent a daughter cell having too many/few chromosomes (called aneuploidy, can lead to congenital defects like Down Syndrome. -
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Chapter 3 - Chapter 3: Hereditary Influences on Development...

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