How do we trace a trait through a family, and why are some traits more common in males?
Analyze pedigrees to determine patterns of inheritance, and explain sex-linked inheritance, including why X-linked recessive traits appear more often in males (GSE SB3.b).
A Georgia Milestones Biology EOC answer on pedigree analysis and sex-linked inheritance: reading pedigree symbols, identifying dominant versus recessive and carriers, the X and Y chromosomes, and why X-linked recessive traits such as color blindness appear more often in males.
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What this topic is asking
Standard SB3.b includes pedigree analysis and sex-linked inheritance. For the Georgia Milestones Biology EOC you must read a pedigree (the family-tree diagram), decide whether a trait is dominant or recessive, identify carriers, and explain sex linkage, especially why X-linked recessive traits show up more often in males. These items reward careful logic from the diagram rather than memorized facts.
Reading a pedigree
The skill is to reason from who is affected to the inheritance pattern, using a few rules.
Dominant or recessive?
A carrier is an individual who is heterozygous, carrying one recessive allele but not showing the trait. Carriers are central to recessive pedigrees, because they pass the allele on silently.
The sex chromosomes
Humans have 23 pairs of chromosomes; the 23rd pair are the sex chromosomes. Females are XX and males are XY. The X chromosome carries many genes unrelated to sex; the small Y chromosome carries few. A gene on the X chromosome is X-linked (sex-linked).
Why X-linked recessive traits favor males
Because a male is XY, he has only one X chromosome and therefore only one copy of any X-linked gene. If that single allele is the recessive one, it is expressed, because there is no second X to mask it. A female is XX, so she has two copies; she only shows an X-linked recessive trait if both of her Xs carry the recessive allele, which is far less likely. A female with one recessive allele is a carrier who can pass it to her sons. This is why traits like red-green color blindness and hemophilia appear more often in males.
Try this
Q1. In a pedigree, what do a square, a circle, and a shaded symbol represent? [3 points]
- Cue. A square is a male, a circle is a female, and a shaded symbol is an affected individual (has the trait).
Q2. Explain why a son inherits an X-linked trait from his mother, not his father. [2 points]
- Cue. A son is XY; he receives his Y from his father and his single X from his mother, so any X-linked allele he has came from his mother.
Exam-style practice questions
Practice questions written in the style of GaDOE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Milestones (style)1 marksColor blindness is an X-linked recessive trait. Why does it appear more often in males than in females? (A) Males have two X chromosomes. (B) Males have only one X chromosome, so a single recessive allele is expressed. (C) The trait is carried on the Y chromosome. (D) Females cannot inherit the allele.Show worked answer →
A 1-point selected-response item on sex-linked inheritance.
The correct answer is B. Males have one X and one Y chromosome (XY), so they have only one copy of an X-linked gene. A single recessive allele on that one X is expressed because there is no second X to mask it. Females (XX) have two X chromosomes, so they need the recessive allele on both Xs to show the trait, which is rarer; a female with one recessive allele is a carrier. A and D are factually wrong, and C misplaces the gene on the Y.
Milestones (style)2 marksIn a pedigree, two unaffected parents have an affected son. Explain what this tells you about whether the trait is dominant or recessive, and define a carrier.Show worked answer →
A 2-point pedigree-reasoning item.
Two unaffected parents producing an affected child means the trait must be recessive: the parents each carried a hidden recessive allele and passed it on, so the child received two recessive alleles and showed the trait. If the trait were dominant, an affected child would need an affected parent. A carrier is an individual who is heterozygous, carrying one recessive allele but not showing the trait, who can still pass the allele to offspring. Full points need the recessive conclusion with reasoning and the definition of a carrier.
Related dot points
- Use Mendel's laws of segregation and independent assortment, with Punnett squares, to predict the genotype and phenotype ratios and probabilities of monohybrid crosses (GSE SB3.b).
A Georgia Milestones Biology EOC answer on inheritance: alleles, genotype and phenotype, dominant and recessive traits, Mendel's laws, and using Punnett squares to predict the ratios and probabilities of monohybrid crosses.
- Use mathematical models to predict and explain patterns of inheritance beyond simple dominance, including incomplete dominance, codominance, and multiple alleles (such as ABO blood type) (GSE SB3.b).
A Georgia Milestones Biology EOC answer on non-Mendelian inheritance: incomplete dominance (blended phenotype), codominance (both alleles shown), and multiple alleles with the ABO blood type system, including how to work out blood-type crosses.
- Explain the role of meiosis in producing gametes and in generating genetic variation through crossing over and independent assortment (GSE SB3.a).
A Georgia Milestones Biology EOC answer on meiosis: how it halves the chromosome number to make gametes, the difference from mitosis, and how crossing over, independent assortment, and random fertilization create genetic variation.
- Compare the advantages and disadvantages of sexual and asexual reproduction, relating genetic variation to survival in stable versus changing environments (GSE SB3.c).
A Georgia Milestones Biology EOC answer comparing sexual and asexual reproduction: the genetic variation of sexual reproduction versus the speed and identical offspring of asexual reproduction, and which is favored in stable versus changing environments.
- Construct an argument that mutations (changes in DNA sequence and chromosomal alterations) may result in phenotypic variation, and classify gene mutations as beneficial, harmful, or neutral (GSE SB2.b).
A Georgia Milestones Biology EOC answer on mutations: point mutations (substitution, insertion, deletion), frameshift effects, chromosomal mutations, causes (mutagens and replication errors), and how mutations can be beneficial, harmful, or neutral sources of variation.
Sources & how we know this
- Biology Georgia Standards of Excellence (GSE) — Georgia Department of Education (2024)
- Georgia Milestones Biology EOC Assessment Guide — Georgia Department of Education (2024)