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Sex Determination & X-Linked Traits Biology 30 ~ Genetics Unit Mrs. S. Pipke-Painchaud.

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Presentation on theme: "Sex Determination & X-Linked Traits Biology 30 ~ Genetics Unit Mrs. S. Pipke-Painchaud."— Presentation transcript:

1 Sex Determination & X-Linked Traits Biology 30 ~ Genetics Unit Mrs. S. Pipke-Painchaud

2 Sex Determination Most higher plants and animals have a pair of sex chromosomes and a certain number of paired autosomes.  In most cases: Males = XY Females = XX

3 Example: Humans Have 23 pairs of chromosomes  n = 23 2n = 46 Humans have 22 autosomes (non sex chromosomes) and 1 pair of sex chromosomes.

4 Diploid numbers of some commonly studied organisms (as well as a few extreme examples) Homo sapiens (human)46 Mus musculus (house mouse)40 Drosophila melanogasterDrosophila melanogaster (fruit fly)8 Caenorhabditis elegansCaenorhabditis elegans (microscopic roundworm)12 Saccharomyces cerevisiaeSaccharomyces cerevisiae (budding yeast)32 Arabidopsis thalianaArabidopsis thaliana (plant in the mustard family)10 Xenopus laevis (South African clawed frog)36 Canis familiaris (domestic dog)78 Gallus gallus (chicken)78 Zea mays (corn or maize)20 Muntiacus reevesi (the Chinese muntjac, a deer)23 Muntiacus muntjac (its Indian cousin)6 Myrmecia pilosula (an ant)2 Parascaris equorum var. univalens (parasitic roundworm)roundworm2 Cambarus clarkii (a crayfish)200 Equisetum arvense (field horsetail, a plant)horsetail216 All info exactly from Chromosomes:

5 What is the chance of having a baby girls when the parents are expecting a child? There is always a 1 in 2 chance of having a boy or a girl.  Females after meiosis produce a gamete with 22 autosomes and 1 X chromosome. Since females only produce 1 type of gamete they are called homogametic  Males after meiosis produce a gamete with 22 autosomes and either an X or a Y chromosome. (both the X and the Y occur with equal frequency) Since males produce 2 types of gametes they are called heterogametic.

6 Ex/ XX x XY XY XXXXY XXXXY Genotypic Ratio: 1 XX: 1 XY Phenotypic Ratio: 1 Female: 1 Male

7 During Meiosis... The X and Y chromosomes behave as a homologous pair. XY = (draw in chromosomes) Part of the Y chromosome carries different genes.

8 X ChromosomeY Chromosome

9 What makes a male a male? The lack of a second X or the presence of a Y?  The presence of a Y

10 Sex Determination Chart Not every animal has the same sex chromosomes. TypeEX.MaleFemaleHomogameticHeterogametic XYHumans & Fruit Flies XYXXFM 2N/ NBeesNNNM & F-- XOGrasshopperXOXXFM ZWBirdsZZZWMF ZOChickensZZZOMF

11 In Human Males... XY – whichever allele is on the X chromosome will be directly expressed because there is no other common allele.  Called HEMIZYGOUS  **When looking at pedigrees with X-Linked traits, they exhibit a criss-cross pattern of inheritance.

12 Are You Color Blind?

13 Color Blindness Example X C = Normal X c = Color Blind

14 Mother passes the traits to all the sons and not the daughters because the sons are hemizygous Offspring marry normal individuals Color blind sons produce normal offspring Normal daughters produce normal daughters and color blind sons Males always inherit the sex linked traits from their mother. Females inherit an X chromosome from each parent; therefore, inherit sex linked traits from both.

15 Pedigree Hints: Refer to examples on Sex Linkage: Chap02/sex_link.html Chap02/sex_link.html

16 X-Linked Trait Examples Red Green Color Blindness Duchenne’s Muscular Dystrophy Hemophilia Calico Cats Tortise Shell Cats Images from Wikipedia: http://en.wik iki/Sex_link age

17 Challenge: A drone bee (male) has no father but does have a grandfather. Why?  Drone’s job (never works) is to fertilize the queen  He has no stinger and cannot defend himself  The hive tolerates the drone until food is short and then the female workers sting the drones to death and throw them out  Drones mate with the queen  Queen lays eggs in special chambers  As they do the sperm fertilizes each individual egg (i.e. one at a time)  When the Queen lays the last few eggs there is no sperm left Fertilized eggs = 2N = female Not fertilized = N = male drones

18 X-Linked Traits Aka Sex Linked Traits In humans, there are many genes found on the X chromosome that are not on the Y chromosome. In Drosophila, normal eye color is red and the recessive eye color is white.

19 Ex/ Drosophila (Fruit Flies) X W = Normal Red Eyes X w = White Eyes  If the gene occurs only on the X chromosome, then there are only 3 possible gamete combinations.  1) X W 2) X w 3) Y

20 X W = Normal Red Eyes X w = White Eyes  If you cross a white eyed male X a normal red eyed female (homozygous), what would be the phenotypic ratio? Parents: X w Y X X W X W  Hint: Write down XY and XX first and then add the info Genotypic Ratio:  1 X W X w : 1 X W Y Phenotypic Ratio:  1 red eyed female: 1 red eyed male  Image from:  http://www.extension.umn.e du/yardandgarden/YGLNe ws/images/fruitfly.jpg http://www.extension.umn.e du/yardandgarden/YGLNe ws/images/fruitfly.jpg XwXw Y XWXW X W X w X W Y XWXW X W X w X W Y

21 If you crossed a heterozygous red eyed female with a red eyed male, what would the genotypic and phenotypic ratios be?  X W X w X X W Y  Genotypic Ratio: 1 X W X W : 1 X W X w : 1 X W Y: 1 X w Y  Phenotypic Ratio: 2 red eyed females: 1 Red eyed male: 1 white eyed male XWXW Y XWXW X W X W Y XwXw X W X w X w Y

22 Any Questions? Now try the assigned questions and exit slips.

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