Chapter 4 - Sex Determination

why are human males hemizygous?

Males and females have different complements of sex chromosomes. Human females have two X chromosomes, and human males have one X chromosome and one Y chromosome. Therefore, human males are hemizygous for all genes on the X chromosome

intersex

-"between the sexes" -an uncommon individual in a dioecious species -individuals possess rudimentary characteristics of both sexes and are usually sterile

transgenic

-An organism that contains a gene from a different organism

hemizygosity

the condition in which an otherwise diploid organism posses only one copy of a gene

What do the terms monoecious, dioecious, and hermaphroditic refer to?

the distribution of reproductive organs between different individuals in sexually reproductive species

SRY gene

the sex determining region of the Y chromosome in males. Encodes the testis-determining factor, which turns the primordial gonads into the testes

XY with SRY deleted

phenotypically female but karyotype indicates the presence of both sex chromosomes

XXX

phenotypically female with some abnormalities and over-expression of X chromosome genes

XX with SRY on X

phenotypically male but karyotype indicates the presence of only X chromosomes

In certain salamanders, the sex of a genetic female can be altered, changing her into a functional male; these salamanders are called sex-reversed males. When a sex-reversed male is mated with a normal female, approximately 2/3 of the offspring are female and 1/3 are male. Given this observation, which of the following statements is most likely to be true for these salamanders? How do you explain the results of this cross?

-Females are the heterogametic sex. -Both parents were ZW and WW offspring did not survive.

XYY

-Jacobs syndrome -phenotypically male with an increase in average stature

XXY

-Klinefelter syndrome -phenotypically male with sterility and hypogonadism

Nor regularly hemizygous

-a color blindness gene on an X chromosome in a normal human female -a gene on chromosome 21 in an individual with Down Syndrome

Regularly hemizygous

-a color blindness gene on the X chromosome in a normal human male -a transgenic gene inserted into only one chromosome of a mouse

Which statements describe Y‑linked traits?

-can only be passed father to son (females do not have a Y chromosome) -females cannot be carriers -expressed solely in males

In butterflies, sex is determined by the ZW sex‑determination system. Female butterflies are heterogametic and have both a Z sex chromosome and a W sex chromosome for sex determination. In contrast, male butterflies are homogametic and have two Z sex chromosomes. Select all of the relatives from which a female butterfly could have inherited her Z sex chromosome.

-father -paternal grandfather

Down Syndrome (Trisomy 21)

-genetic disorder most commonly caused by the presence of an additional copy of chromosome 21 -affects both males and females -symptoms include mild to moderate intellectual disabilities, stunted growth, low muscle tone, and distinct facial features such as a flat head, flattened nose, and slanted eyes

what is the difference between monoecious and hermaphroditic?

-in animals, monoecious and hermaphroditic mean the same thing -in plants, there is a slight difference.

dioecious

-individual is either male or female, but not both -belong to species in which the sexes are always separate

what is the difference between hermaphroditic and intersex?

-intersex individuals possess rudimentary characteristics of both sexes and are usually sterile -hermaphroditic individuals are fertile and have fully functional gonads that produce both male and female gametes

monoecious

-male and female reproductive structures occur in the same individual

X0

-phenotypically female -Turner Syndrome

What is dosage compensation?

-refers to the phenomenon that the level of expression of genes on the sex chromosomes is similar in males and females, even though they have different numbers of sex chromosomes. In many species it seems necessary so that the balance of gene expression between the autosomes and sex chromosomes is similar between the two sexes. -the unequal dosage between the two sexes is rectified by either increasing the transcription of genes on the X chromosome in males or decreasing the transcription of genes on the X chromosome in females

Two model organisms for dosage compenstation

1) Drosophlia -the transcription of genes on the X chromosome in males is increased to a level equal to the sum of the transcriptional level of both X chromosomes in females 2) C. elegans -the transcription of genes on the X chromosome in females is decreased so that the sum of the transcriptional level of the two X chromosomes is the same as that of the single X chromosome in males

hypogonadism

decrease or lack of hormones normally produced by the gonads

Fruit flies are sexually reproducing insects with separate male and female individuals. Like most mammals, including humans, males have an X and a Y chromosome, and females have two X chromosomes. Select the term that describes the organization of the sexual organs in the fruit fly.

dioecious

In C. elegans, the level of expression of genes on both X chromosomes in females is reduced by half. What phenomenon is this an example of?

dosage compensation

Pine trees have male and female cones on the same tree. Which term describes the sex system of the organism?

monoecious