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Chapter 4: Problem 17

For each of the following chromosome complements, what is the phenotypic sex of a person who has a. XY with the \(S R Y\) gene deleted? b. XX with a copy of the SRY gene on an autosomal chromosome? c. XO with a copy of the \(S R Y\) gene on an autosomal chromosome? d. XXY with the SRY gene deleted? e. XXYY with one copy of the \(S R Y\) gene deleted?

Short Answer

Expert verified
a. Phenotypically female; b. Phenotypically male; c. Phenotypically male; d. Phenotypically female; e. Phenotypically male.

Step by step solution

01

Understand the Role of SRY Gene

The SRY (Sex-determining Region Y) gene is critical for determining maleness in humans. It is typically located on the Y chromosome and triggers the development of testes, leading to male phenotypic characteristics. In the absence of the SRY gene, the default developmental pathway is female.
02

Analyze XY with SRY Deletion

In an XY individual lacking the SRY gene, the primary driver for male development is absent. Without the SRY gene, even though the individual has an X and a Y chromosome, they would develop as phenotypically female because the necessary trigger for male phenotype is missing.
03

Analyze XX with Autosomal SRY

An XX individual has two X chromosomes. However, if a copy of the SRY gene is present on an autosome, it will lead to the development of testes and male characteristics, resulting in a phenotypic male despite the XX chromosome pattern.
04

Analyze XO with Autosomal SRY

An XO individual typically has Turner syndrome, characterized by a single X chromosome. If the SRY gene is located on an autosome, it will provide the trigger necessary for male development, resulting in a phenotypic male.
05

Analyze XXY with SRY Deletion

An XXY individual usually displays Klinefelter syndrome, which often results in male phenotypic characteristics due to the presence of the SRY gene. However, if the SRY gene is deleted, the developmental pathway defaults to female, resulting in a phenotypic female.
06

Analyze XXYY with One SRY Deleted

In an XXYY individual, having two Y chromosomes suggests the presence of two SRY genes. With one SRY gene deleted, one functioning SRY gene remains, which is sufficient to direct male development, resulting in a phenotypic male.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

SRY Gene
The SRY gene, known as the Sex-determining Region Y gene, plays a pivotal role in sex determination in humans. This gene is typically found on the Y chromosome. It acts as a switch, turning on during embryonic development to begin the process of testis formation. Without this gene, the default pathway for human development is to develop as a female.
  • The presence of an active SRY gene leads to the development of male characteristics.
  • The absence of the SRY gene results in the default development as female.
Understanding the function of this gene is crucial, especially in genetic conditions where its behavior can differ from the norm, such as being deleted or misplaced.
Chromosome Complement
The term 'chromosome complement' refers to the complete set of chromosomes a person has. In humans, this typically includes 46 chromosomes: 44 autosomes and 2 sex chromosomes, which can be XX or XY. The arrangement of these chromosomes determines the genetic makeup and can significantly influence an individual's physical traits, including their phenotypic sex.
  • Typical male chromosome complement: XY
  • Typical female chromosome complement: XX
Alterations in the usual chromosome complement can lead to variations like XXY or XO, which are associated with specific syndromes and differences in phenotypic sex expression.
Phenotypic Sex
Phenotypic sex refers to the physical and biological characteristics of an individual that are typically expressed as male or female features. While genetic factors heavily influence phenotypic sex, it is also determined by the presence and action of certain key genes like the SRY gene.
  • SRY gene present: leads to male phenotypic traits
  • SRY gene absent: leads to female phenotypic traits
The phenotypic sex of an individual might not always align with their chromosomal sex due to variations in their genetic toolkit, such as gene deletions or the presence of additional genes on autosomes.
Turner Syndrome
Turner syndrome is a condition that arises when a female is born with only one X chromosome (referred to as XO). Those with Turner syndrome typically develop with female physical characteristics, as there is no Y chromosome to carry the SRY gene.
  • Characteristics: Short stature, delayed puberty, infertility
  • Commonly identified in females
If an SRY gene is found on another chromosome in an XO individual, this can lead to the development of male physical traits, showing how critical this gene is for determining sex differences.
Klinefelter Syndrome
Klinefelter syndrome occurs in individuals with an additional X chromosome, resulting in the XXY chromosome complement. Despite the presence of two X chromosomes, the typical expression of SRY on the Y chromosome leads to male characteristics, albeit with some differences due to the extra DNA.
  • Common features include small testes, reduced fertility, and in some cases, breast development.
  • Though primarily presenting as male, there can be variations in physical and reproductive development.
The presence of the SRY gene typically ensures male development in XXY individuals, but its deletion can tip the balance towards female phenotypic traits, underscoring its importance in sex determination.

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Most popular questions from this chapter

The Talmud, an ancient book of Jewish civil and religious laws, states that, if a woman bears two sons who die of bleeding after circumcision (removal of the foreskin from the penis), any additional sons that she has should not be circumcised. (The bleeding is most likely due to the Xlinked disorder hemophilia.) Furthermore, the Talmud states that the sons of her sisters must not be circumcised, whereas the sons of her brothers should be. Is this religious law consistent with sound genetic principles? Explain vour answer.

Explain how Bridges's study of non disjunction in Drosophila helped prove the chromosome theory of inheritance.

Miniature wings in Drosophila result from an X-linked allele (X") that is recessive to the allele for long wings \(\left(X^{+}\right) .\) Sepia eyes are produced by an autosomal allele ( \(s\) ) that is recessive to an allele for red eyes \(\left(s^{+}\right)\) a. A female fly that has miniature wings and sepia eyes is crossed with a male that has normal wings and is homozygous for red eyes. The \(\mathrm{F}_{1}\) flies are intercrossed to produce the \(\mathrm{F}_{2}\). Give the phenotypes, as well as their expected proportions, of the \(\mathrm{F}_{1}\) and \(\mathrm{F}_{2}\) flies. b. A female fly that is homozygous for normal wings and has sepia eyes is crossed with a male that has miniature wings and is homozygous for red eyes. The \(\mathrm{F}_{1}\) flies are intercrossed to produce the \(\mathrm{F}_{2}\). Give the phenotypes, as well as their expected proportions, of the \(\mathrm{F}_{1}\) and \(\mathrm{F}_{2}\) flies.

A Drosophila mutation called singed (s) causes the bristles to be bent and misshapen. A mutation called purple \((p)\) causes the fly's eyes to be purple in color instead of the normal red. Flies homozygous for singed and purple were crossed with flies that were homozygous for normal bristles and red eyes. The \(\mathrm{F}_{1}\) were intercrossed to produce the \(\mathrm{F}_{2}\), and the following results were obtained. Cross \(\mathrm{P} \quad\) male, singed bristles, purple eyes \(\times\) female, normal bristles, red eyes \(\mathrm{F}_{1} \quad 420\) female, normal bristles, red eyes 426 male, normal bristles, red eyes \(\mathrm{F}_{2} \quad 337\) female, normal bristles, red eyes 113 female, normal bristles, purple eyes 168 male, normal bristles, red eyes 170 male, singed bristles, red eyes 56 male, normal bristles, purple eyes 58 male, singed bristles, purple eyes Cross 2 female, singed bristles, purple eyes \(\times\) male, normal bristles, red \(\mathrm{P} \quad\) eyes \(\mathrm{F}_{1} \quad 504\) female, normal bristles, red eyes 498 male, singed bristles, red eyes \(\mathrm{F}_{2}\) 227 female, normal bristles, red eyes 223 female, singed bristles, red eyes 225 male, normal bristles, red eyes 225 male, singed bristles, red eyes 78 female, normal bristles, purple eyes 76 female, singed bristles, purple eyes 74 male, normal bristles, purple eyes 72 male, singed bristles, purple eyes a. What are the modes of inheritance of singed and purple? Explain your reasoning. b. Give genotypes for the parents and offspring in the \(\mathrm{P}, \mathrm{F}_{1},\) and \(\mathrm{F}_{2}\) generations of cross 1 and cross 2

In certain salamanders, the sex of a genetic female can be altered, changing her into a functional male; these salamanders are called sexreversed males. When a sex-reversed male is mated with a normal female, approximately 23 of the offspring are female and 13 are male. How is sex determined in these salamanders? Explain the results of this
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