Why do you share some but not all characters of each parent?
Post on 13-Jan-2016
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Heredity• Passing of
genetic traits from parent to offspring
Chromosome theory of heredity• Chromosomes
carry genes• Gene = unit of
heredity
Somatic Cells are “body” cells and contain the normal number of chromosomes . Examples would be … skin cells, brain cells, etc. Diploid
Gametes are the “sex” cells and contain only ½ the normal number of chromosomes. Haploid Sperm cells and ova are gametes.
Pair of chromosomes Pair of chromosomes (maternal and paternal) similar in shape and size.
Each locuslocus (position of a gene) is in the same position on homologues.
Humans have Humans have 23 pairs 23 pairs of of homologous chromosomeshomologous chromosomes..• 22 pairs of autosomespairs of autosomes• 1 pair of sex chromosomespair of sex chromosomes
Homologous pairs Homologous pairs (tetrads are duplicated) carry genes controlling the same inherited traits.
PaternalPaternal MaternalMaternal
eye coloreye color locuslocus
eye coloreye color locuslocus
hair colorhair color locuslocus
hair colorhair color locuslocus
• In Humans the In Humans the “Autosomes” “Autosomes” are sets 1 – 22are sets 1 – 22
• (The Autosomes (The Autosomes code for most of code for most of the offspring’s the offspring’s traits)traits)
XX chromosome - femaleXX chromosome - female XY chromosome - maleXY chromosome - male
In Humans the “Sex In Humans the “Sex Chromosomes” are Chromosomes” are
the 23the 23rdrd set set
Reproduction where the genetic material combined is called sexual reproduction
Two cells, a sperm and an egg, unite to form a zygote, the single cell from which the organism develops
Meiosis is the process of producing sperm and eggs (gametes) – the number of chromosomes are halved
1) Meiosis takes a cell with 1) Meiosis takes a cell with two copies of every two copies of every chromosome (chromosome (diploiddiploid) and ) and makes cells with a single makes cells with a single copy of every chromosome copy of every chromosome ((haploidhaploid). ).
In meiosis, one diploid cells In meiosis, one diploid cells produces produces four haploid cellsfour haploid cells..
2) Meiosis scrambles the genes that each sex cell (egg or sperm) receives.
This makes for a lot of genetic diversity. This happens through independent assortment and
crossing-over. Genetic diversity is important for the evolution of
populations and species.
MeiosisMeiosisParent cell – chromosome pair
Chromosomes copied
1st division - pairs split
2nd division –4 gamete cells with ½ the original number of chromosomes
Gametes have exactly one set of chromosomes, this state is called haploid (1n)
Regular cells have two sets of chromosomes, this state is called diploid (2n)
Why?
Meiosis – mouse testes
Parent cellParent cell
4 gametes4 gametes
11stst division division
22ndnd division division
Similar to mitosis interphase.
Chromosomes replicate (S phase).
Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres.
Centriole pairs also replicate.
The Key Difference Between Mitosis and Meiosis is the Way Chromosomes
Uniquely Pair and Align in Meiosis
Mitosis
Meiosis 1st division
Unlike mitosis, homologous chromosomes line up next to each other during prophase
This process is called synapsing
Lined up homologues are called tetrads
Longest and most complex phase.
90% of the meiotic process is spent in Prophase I
Chromosomes condense. Synapsis occurs:
homologous chromosomes come together to form a tetrad.
Tetrad is two chromosomes or four chromatids (sister and non-sister chromatids).
centriolesspindle fiber
homologues
Prophase 1– Crossing Over
Homologous chromosomes connect at areas called chiasmata
Segments break and reform at similar locations.
This results in new genetic combinations
metaphase plate
OR
metaphase plate
Shortest phaseTetrads align on the metaphase plate.
INDEPENDENT ASSORTMENT
Independent AssortmentIndependent Assortment
Independent assortment produces 2n distinct gametes, where n = the number of unique chromosomes.
In humans, n = 23 223 = 6,000,0000.
That’s a lot of diversity by this mechanism alone
Homologous chromosomes separate and move towards the poles.
Sister chromatids remain attached at their centromeres.
Each pole now has haploid set of chromosomes.
Cytokinesis occurs and two haploid daughter cells are formed.
same as prophase in mitosissame as prophase in mitosis spindle reforms and chromosomes spindle reforms and chromosomes
move toward the metaphase platemove toward the metaphase plate
same as metaphase in mitosissame as metaphase in mitosis sister chromatids lined up on the metaphase platesister chromatids lined up on the metaphase plate
metaphase platemetaphase plate
same as anaphase in mitosissame as anaphase in mitosis sister chromatids are separated and
pulled toward opposite poles of the cell
Same as telophase in mitosisSame as telophase in mitosisNuclei form, Cytokinesis occurs.Nuclei form, Cytokinesis occurs.Four haploid daughter cells produced.Four haploid daughter cells produced.
At least 6 million combinations from Mum and another 6 million from Dad …
>36 trillion combinations for a diploid zygote!!!
In males is called spermatogenesisspermatogenesis and produces sperm.
In females is called oogenesisoogenesis and produces ova.
2n=46
n=23
n=23
meiosis I
n=23
sperm
meiosis II
4 sperm cells are produced from each primary spermatocyte
Primary Spermatocyt
e
Secondary Spermatocyte
n=23
n=23
n=23
The male gamete is called a spermatozoa Four spermatocytes are formed during meiosis Men are busy - meiosis produces roughly
250,000 sperm a day.
The female gamete is called an ovum
At birth each female carries a lifetime supply of developing oocytes, each of which is in Prophase I.
A developing egg (secondary oocyte) is released each month from puberty until menopause, a total of 400-500 eggs.
Only one ovum is produced during meiosisOogenesis places most of the cytoplasm into the large egg. The other cells, the polar bodies, do not develop.
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