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Crossing over Linkages Sex linkages

Crossing over Linkages Sex linkages. How to shuffle the genes. Polygenic Inheritance. So far we’ve only looked at traits that are caused by 1 gene. Some traits are caused by 2 or more genes. This is called polygenic inheritance. Eye colour ,skin colour, and hair colour are all polygenic.

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Crossing over Linkages Sex linkages

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  1. Crossing overLinkagesSex linkages How to shuffle the genes

  2. Polygenic Inheritance • So far we’ve only looked at traits that are caused by 1 gene. • Some traits are caused by 2 or more genes. • This is called polygenic inheritance. • Eye colour ,skin colour, and hair colour are all polygenic.

  3. A quick reminder • Genes are segments of DNA • They are a series of base pairs • 1 gene codes for 1 protein • We need many proteins we use them for enzymes, hormones , structural body parts, pigments, lots of stuff. • Each chromosome has hundreds of genes • Each gene has a special place like beads on a string.

  4. The linkage • Each chromosme has hundreds of genes on it. • Each gene is found in a certain place or locus • If the two genes we are looking at are on the same chromosome then they are linked.

  5. Linked genes and the Law of Independent Assortment • If two genes are on different chromosomes they sort independently and inherited combinations show up in a Phenotypic ratio of 9: 3: 3: 1 • If two genes are one the same chromosome they are linked and give a phenotypic ratio of 3:1 just like a monohybrid cross.

  6. The tetrad • In Metaphase 1 the two sets of homologous chromosomes line up at the equator of the cell. This is called a tetrad because there are 4 side by side.

  7. The chiasma • Sometimes the chromosomes are sticky and the homologous pairs in the middle cross over each other • This is called A chiasma • The DNA can Switch chromosomes And shuffle the genes.

  8. Crossing overorRecombination • When there is an equal trade between non- sister chromotids at the chiasma you can get new combinations of genes

  9. How we can map chromosomes • The farther apart genes are on a chromosome the more likely they are to cross over. • We count the frequency of crossovers between all the genes on a chromosome and find out their relative positions.

  10. Sex chromosomes and Autosomes • You have 23 pairs of chromosomes, one from your mom and one from your Dad. 22 pairs are homologous. These are your autosomes. The 23rd pair is not homologous. They determine your sex. If you have XX chromosomes you are a girl. XY you are a boy. The Y is very small and doesn’t carry any genes other than sex determination. These are your sex chromosomes.

  11. Sex linkages The last pair are your sex chromosomes. They aren’t the same size X is bigger and carries more genes. The y is small and doesn’t have many genes. In humans the Y determines the sex. If the gene in question is on the X chromosome we call that SEX-Linked. It poses and interesting problem. The male has only 1 X. He either gets the dominant or the recessive. He only gets one chance. This is called hemizygous.

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