A woman has a condition where all of her gametes undergo nondisjunction of chromosome 21 in meiosis II, but meiosis I proceeds normally. She mates with a man who produces all normal gametes. What is the probability that the fertilized egg will develop into a child with Down syndrome (trisomy 21)

1/4.

Explanation:

If the nondisjunction occurs in meiosis II, 4 gametes are formed in which types of gametes are 3. Out of the 4 gametes, 2 are normal, 2 will have aneuploidy as shown in the diagram attached.  Out of the 2 aneuploid gametes, one will have 2 haploid chromosomes while another one will not have any chromosome at all.

Now when this female will mate with a man who has produced normal gametes, each of the gamete produced by this female will have the probability to receive 1 haploid gamete from the male.  After fusion/fertilization, total 4 zygotes which are of 3 types may form. One of the zygotes will have 3 chromosomes i.e. will have trisomy (which will develop into Down's syndrome), one of them will have 1 chromosome i.e. will have monosomy while 2 zygotes will be normal i.e. will be diploid.

So, the probability of a child being born with Down's syndrome will be 1/4.

hi there!

answer: b. nucleolus

explanation: nucleolus is a structure in the nucleus are ribosomes are made.

hope this you!

have a nice day! : )

: d

-charlie

you! : )

: d

an individual’s sex (i.e., whether they are a male or female) is determined by the sex chromosomes. most people have two sex chromosomes, one that is inherited from their mother and one that is inherited from their father. typically, females have two x chromosomes (xx) and males have one x chromosome and one y chromosome (xy). conditions caused by changes (“mutations”) in genes located on the x chromosome are considered x-linked.

x-linked recessive inheritance

most x-linked conditions are recessive. this means that in a person with two x chromosomes (most females), both copies of a gene (i.e., one on each x chromosome) must have a change or mutation whereas in a person with one x chromosome (most males), only one copy of a gene must have a mutation. a female with a mutation in one copy of a gene on the x chromosome is said to be a “carrier” for an x-linked condition. a male with a mutation in a gene on the x chromosome is typically affected with the condition. because females have two copies of the x chromosome and males have only one x chromosome, x-linked recessive diseases are more common among males than females. however, x-linked recessive diseases can occur in both males and females.

for x-linked recessive disorders, an unaffected carrier mother who has a mutation in a gene on the x chromosome can transmit either the x chromosome with this mutation or a “normal” x chromosome to her children. if the father is unaffected, none of her daughters will be affected and all of her daughters will be unaffected—since they will inherit at least one normal x chromosome from their father. however, each daughter will have a 50% chance of being an unaffected carrier like her mother and a 50% chance of both x chromosomes being normal.